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