msm: ext-buck-support: add external buck support driver

Based on the dt file entry enables the external buck support
for sending active set and sleep set votes to turn-on and turn-off
external buck by toggling a gpio when the system is going PC.

CRs-fixed: 568380
Change-Id: I8babf39f71a94daea2ba256d677f9a9f2bb77c30
Signed-off-by: Anil kumar mamidala <amami@codeaurora.org>