regulator: msm_gfx_ldo: specify a list_corner_voltage callback function

Define a list_corner_voltage callback function for the msm_gfx_ldo
driver so that consumers can determine the msm_gfx_ldo voltage
for each voltage corner.

CRs-Fixed: 2174431
Change-Id: I3f661f9bbb01bcce4b7f56707583eea507045b47
Signed-off-by: Tirupathi Reddy <tirupath@codeaurora.org>