msm: gdsc: Introduce an API for hardware triggered GDSC control

The GDSC can optionally be triggered by hardware as well as software.
Previously, the GDSC regulator API allowed only software triggering.
HW triggering may potentially lead to greater power savings. Use
regulator_set_mode to provide a way to enable hardware triggering
as well.

Change-Id: I7f9401ee986db2e1ddf2755484d0d959a6869c21
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>