msm: gdsc: Allow core logic collapse to be disabled via device-tree

Introduce an optional device-tree binding that can be used to
keep the core's logic powered, but still allow its memories
to collapse.

Change-Id: I31a025be4c811c8a133b2e784ea56e65e175bd3a
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>