regulator: gfx-ldo: support voltage based regulator operations

The current gfx ldo driver supports only corner based regulator
operations. The gfx ldo(LDO300) regulator in msmfalcon is CPR
managed and requires support for voltage based regulator operations.
Add voltage based regulator operations for msmfalcon.

CRs-Fixed: 1068294
Change-Id: I510c2fe7f763c8d44c67794c889c687df60398d7
Signed-off-by: Tirupathi Reddy <tirupath@codeaurora.org>