power: pmic-voter: Add API to override votes

Add an API to override votes on a votable. This is useful
in case where one wants to force a value on a MIN/MAX votable
irrespective of its current value.

If two clients vote to override same votable, the latest vote
is set.

Change-Id: Ida14d686905df2c0c3340cc1832ece1bba31b454
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>
Signed-off-by: Sahil Chandna <chandna@codeaurora.org>