dt-bindings: power: qcom,rpmpd: document sa8775p

Add a compatible for sa8775p platforms and relevant defines to the include
file.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230109174511.1740856-14-brgl@bgdev.pl