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>