power: pm8921-charger: add UVD threshold API

PM8917 has a new register which allows configuring
the falling USB under voltage detection (UVD) threshold
from 3.85V to 4.35V in 100 mV steps.

Add an API that allows to set this value per default and
also only do this for PM8917 chips.

Change-Id: I9b4259b43f148762ac72af40bdc79b895d9b4fc2
Signed-off-by: David Keitel <dkeitel@codeaurora.org>