bq20z75: Add support for charge properties

Adding support for charge properties for gas gauge.

Also ensuring that battery mode is correct now for energy as well as
charge properties by setting it on the fly.

I also added 2 functions to power_supply.h to help identify the units for
specific properties more easily by power supplies.

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