msm: gdsc: Add support for enabling memory retention for Oxili GFX3D

Add memory and periphery state retention support for GFX3D on
8974. When enabled, this will prevent any core memory state from
being lost when the core's clock is gated off. This feature
can be enabled by setting the "qcom,retain-mems" in the device
tree.

Change-Id: I7f3bfb8efaa7982e36f4e1c4f4173282b204a898
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>