pmic-voter: move to common header location

pmic-voter.h needs to be used in the qcom-tadc driver which resides
outside of the power/supply/qcom location. Move it to include/linux.

Change-Id: I5079a8504573da2a800fd458ef9aa5931056d8c2
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>