dt-bindings: sram: Document qcom,rpm-msg-ram

The Qualcomm SMEM binding always depended on a reference to a SRAM node
of compatible "qcom,rpm-msg-ram", document this as part of the SRAM
binding.

The SRAM is consumed as a whole and not split up using subnodes, so
properties related to this are not required.
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
Link: https://lore.kernel.org/r/20210930182111.57353-2-bjorn.andersson@linaro.org

Documentation/devicetree/bindings/sram/sram.yaml

@@ -31,6 +31,7 @@ properties:
         - amlogic,meson-gxbb-sram
         - arm,juno-sram-ns
         - atmel,sama5d2-securam
+        - qcom,rpm-msg-ram
         - rockchip,rk3288-pmu-sram
 
   reg:
@@ -135,7 +136,9 @@ if:
   properties:
     compatible:
       contains:
-        const: rockchip,rk3288-pmu-sram
+        enum:
+          - qcom,rpm-msg-ram
+          - rockchip,rk3288-pmu-sram
 
 else:
   required: