pmic-voter: add support to report override votes

Add support to report if a votable is voted via override
voting mechanism.
While at it, add valid pointer checks in global voter functions.

Change-Id: Ib867144c92f4ae99d9391190f75c8a6c95a3f4f0
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>