dt-bindings: mmc: Set maximum documented operating frequency as 384MHz

As Ulf noted in [1], the maximum operating frequency
documented in the mmc-controller device-tree bindings
should be updated to the maximum frequency supported
by the mmc controller(s).

Without this fix in place, the 'make dtbs_check' reports
issues with 'max-frequency' value for ipq8074 sdhci node:

  arch/arm64/boot/dts/qcom/ipq8074-hk01.dtb: mmc@7824900:
   max-frequency:0:0: 384000000 is greater than the maximum of 200000000

[1]. https://www.spinics.net/lists/kernel/msg4442049.html

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220726084520.2895454-1-bhupesh.sharma@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/mmc-controller.yaml

@@ -88,11 +88,18 @@ properties:
     default: 1
 
   max-frequency:
-    description:
-      Maximum operating frequency of the bus.
+    description: |
+      Maximum operating frequency of the bus:
+        - for eMMC, the maximum supported frequency is 200MHz,
+        - for SD/SDIO cards the SDR104 mode has a max supported
+          frequency of 208MHz,
+        - some mmc host controllers do support a max frequency upto
+          384MHz.
+      So, lets keep the maximum supported value here.
+
     $ref: /schemas/types.yaml#/definitions/uint32
     minimum: 400000
-    maximum: 200000000
+    maximum: 384000000
 
   disable-wp:
     $ref: /schemas/types.yaml#/definitions/flag