Documentation: mmc: sdhci-of-arasan: Add soc-ctl-syscon for corecfg regs

As can be seen in Arasan's datasheet [1] there are several "corecfg"
settings in their SDHCI IP Block that are supposed to be controlled by
software.  Although the datasheet referenced is a bit vague about how to
access corecfg, in Figure 5 you can see that for Arasan's PHY (a
separate component than their SDHCI component) they describe the
"phyctrl" registers as being "FROM SOC CTL REG", implying that it's up
to the licensee of the Arasan IP block to implement these registers.  It
seems sane to assume that the "corecfg" registers in their SDHCI IP
block works in a similar way for all licensees of the IP Block.

Device tree has a model that allows a device to get a reference to
random registers located elsewhere in the SoC: sysctl.  Let's leverage
this model and allow adding a sysctl reference to access the control
registers for the Arasan SDHCI PHYs.

Having a reference to the control registers doesn't do much for us on
its own since the Arasan spec doesn't specify how these corecfg values
are laid out in memory.  In the SDHCI driver we'll need a map detailing
where each corecfg can be found in each implementation.  This map can be
found using the primary compatible string of the SDHCI device.  In that
spirit, document that existing rk3399 device trees already have a
specific compatible string, though up to now they've always been relying
on the driver supporting the generic.

Note that since existing devices seem to work fairly well as-is, we'll
list the syscon reference as "optional", but it's likely that we'll run
into much fewer problems if we can actually set the proper values in the
syscon, so it is strongly suggested that any SoCs where we have a map to
set the corecfg also include a reference to the syscon.

[1]: https://arasan.com/wp-content/media/eMMC-5-1-Total-Solution_Rev-1-3.pdf

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/arasan,sdhci.txt

@@ -9,8 +9,12 @@ Device Tree Bindings for the Arasan SDHCI Controller
   [4] Documentation/devicetree/bindings/phy/phy-bindings.txt
 
 Required Properties:
-  - compatible: Compatibility string. Must be 'arasan,sdhci-8.9a' or
-                'arasan,sdhci-4.9a' or 'arasan,sdhci-5.1'
+  - compatible: Compatibility string.  One of:
+    - "arasan,sdhci-8.9a": generic Arasan SDHCI 8.9a PHY
+    - "arasan,sdhci-4.9a": generic Arasan SDHCI 4.9a PHY
+    - "arasan,sdhci-5.1": generic Arasan SDHCI 5.1 PHY
+    - "rockchip,rk3399-sdhci-5.1", "arasan,sdhci-5.1": rk3399 eMMC PHY
+      For this device it is strongly suggested to include arasan,soc-ctl-syscon.
   - reg: From mmc bindings: Register location and length.
   - clocks: From clock bindings: Handles to clock inputs.
   - clock-names: From clock bindings: Tuple including "clk_xin" and "clk_ahb"
@@ -22,6 +26,11 @@ Required Properties for "arasan,sdhci-5.1":
   - phys: From PHY bindings: Phandle for the Generic PHY for arasan.
   - phy-names:  MUST be "phy_arasan".
 
+Optional Properties:
+  - arasan,soc-ctl-syscon: A phandle to a syscon device (see ../mfd/syscon.txt)
+    used to access core corecfg registers.  Offsets of registers in this
+    syscon are determined based on the main compatible string for the device.
+
 Example:
 	sdhci@e0100000 {
 		compatible = "arasan,sdhci-8.9a";
@@ -42,3 +51,17 @@ Example:
 		phys = <&emmc_phy>;
 		phy-names = "phy_arasan";
 	} ;
+
+	sdhci: sdhci@fe330000 {
+		compatible = "rockchip,rk3399-sdhci-5.1", "arasan,sdhci-5.1";
+		reg = <0x0 0xfe330000 0x0 0x10000>;
+		interrupts = <GIC_SPI 11 IRQ_TYPE_LEVEL_HIGH>;
+		clocks = <&cru SCLK_EMMC>, <&cru ACLK_EMMC>;
+		clock-names = "clk_xin", "clk_ahb";
+		arasan,soc-ctl-syscon = <&grf>;
+		assigned-clocks = <&cru SCLK_EMMC>;
+		assigned-clock-rates = <200000000>;
+		phys = <&emmc_phy>;
+		phy-names = "phy_arasan";
+		status = "disabled";
+	};