dt-bindings: misc: fastrpc: Document memory-region property

Add memory-region property to the list of optional properties, specify
the value type and a definition. This property is used to specify the
memory region which should be used for remote heap CMA.

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220909133938.3518520-4-abel.vesa@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>