dt-bindings: power: Add the bindings for the bq2515x family of chargers.

dt-bindings: power: Add the bindings for the bq2515x family of chargers.

The BQ2515X family of devices are highly integrated battery management
ICs that integrate the most common functions for wearable devices
namely a charger, an output voltage rail, ADC for battery and system
monitoring, and a push-button controller.

Datasheets:
http://www.ti.com/lit/ds/symlink/bq25150.pdf
http://www.ti.com/lit/ds/symlink/bq25155.pdf

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ricardo Rivera-Matos <r-rivera-matos@ti.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>