power_supply: Add driver for TI BQ20Z75 gas gauge IC

This driver depends on I2C and uses SMBUS for communication with
the host.

Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Reviewed-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>