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>