dt-bindings: spi: Adjust the bindings for the FSL QSPI driver

Adjust the documentation of the new SPI memory interface based
driver to reflect the new drivers settings.

The "old" driver was using the "fsl,qspi-has-second-chip" property to
select one of two dual chip setups (two chips on one bus or two chips
on separate buses). And it used the order in which the subnodes are
defined in the dt to select the CS, the chip is connected to.

Both methods are wrong and in fact the "reg" property should be used to
determine which bus and CS a chip is connected to. This also enables us
to use different setups than just single chip, or symmetric dual chip.

So the porting of the driver from the MTD to the SPI framework actually
enforces the use of the "reg" properties and makes
"fsl,qspi-has-second-chip" superfluous.

As all boards that have "fsl,qspi-has-second-chip" set, also have
correct "reg" properties, the removal of this property shouldn't lead to
any incompatibilities.

The only compatibility issues I can see are with imx6sx-sdb.dts and
imx6sx-sdb-reva.dts, which have their reg properties set incorrectly
(see explanation here: [2]), all other boards should stay compatible.

Also the "big-endian" flag was removed, as this setting is now selected
by the driver, depending on which SoC is in use.

[2] https://patchwork.ozlabs.org/patch/922817/#1925445

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-fsl-qspi.txt

@@ -14,15 +14,13 @@ Required properties:
   - clocks : The clocks needed by the QuadSPI controller
   - clock-names : Should contain the name of the clocks: "qspi_en" and "qspi".
 
-Optional properties:
+Required SPI slave node properties:
-  - fsl,qspi-has-second-chip: The controller has two buses, bus A and bus B.
+  - reg: There are two buses (A and B) with two chip selects each.
-                              Each bus can be connected with two NOR flashes.
+	 This encodes to which bus and CS the flash is connected:
-			      Most of the time, each bus only has one NOR flash
+		<0>: Bus A, CS 0
-			      connected, this is the default case.
+		<1>: Bus A, CS 1
-			      But if there are two NOR flashes connected to the
+		<2>: Bus B, CS 0
-			      bus, you should enable this property.
+		<3>: Bus B, CS 1
-			      (Please check the board's schematic.)
-  - big-endian : That means the IP register is big endian
 
 Example:
 
@@ -40,7 +38,7 @@ qspi0: quadspi@40044000 {
 	};
 };
 
-Example showing the usage of two SPI NOR devices:
+Example showing the usage of two SPI NOR devices on bus A:
 
 &qspi2 {
 	pinctrl-names = "default";