bq20z75: Add support for more power supply properties

This patch adds properties to support devicekit power. Also, create IO
wrapper functions and fix some issues found while testing, including
unit conversions to match the power_supply types.

Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>