arm/dt: msm8974: gdsc: Enable Oxili GFX3D memory retention

Set the "qcom,retain-mems" property so that the Oxili GFX3D memories
and periphery remained powered and their contents retained during
clock gating and GDSC collapse.

Change-Id: Id64681e013ef17632dc7d30147fe14d786731da1
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>