power_supply: Add TI BQ24257 charger driver

Based on the datasheet found here:
http://www.ti.com/lit/ds/symlink/bq24257.pdf



Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>