dt-bindings: interrupt-controller: mpm: Pass MSG RAM slice through phandle

Due to the wild nature of the Qualcomm RPM Message RAM, the kernel can't
really use 'reg' to point to the MPM's slice of Message RAM without cutting
into an already-defined RPM MSG RAM node used for GLINK and SMEM.

Document passing the register space as a slice of SRAM through the
qcom,rpm-msg-ram property. This also makes 'reg' deprecated.

Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230328-topic-msgram_mpm-v7-1-6ee2bfeaac2c@linaro.org