Add APIs to get and set the voltage for the target regulator.
BUG=b:147789962
BRANCH=none
TEST=emerge-asurada coreboot
Change-Id: I0e56df45fc3309c387b9949534334eadefb616b2
Signed-off-by: Yidi Lin <yidi.lin@mediatek.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46404
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>