msm: gdsc: Separate CORE and PERIPH memory retention control

Currently, a single device tree property determines whether core
memory and peripheral memory should be collapsed along with the
core's logic. For greater flexibility, split this into two device
tree properties.  The functionality that was previously part of
'qcom,retain-mems' is split into 'qcom,retain-mem' and
'qcom,retain-periph'.

Current users of 'qcom,retain-mems' are updated accordingly.

Change-Id: I5a77cab5cda5482aa22b9d19b77e4816b0324eca
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>