dt-bindings: firmware: qcom,scm: add memory-region for sa8775p

Document a new property (currently only for sa8775p) that describes the
memory region reserved for communicating with the TrustZone.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20240527-shm-bridge-v10-1-ce7afaa58d3a@linaro.org


Signed-off-by: Bjorn Andersson <andersson@kernel.org>