soc: qcom: smd-rpm: Add MSM8976 compatible

Add a compatible for the RPM on the Qualcomm MSM8976 platform:
this is also valid for MSM8956 and their APQ variants.

Signed-off-by: Angelo G. Del Regno <kholk11@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>