dt-bindings: power: Add the bq256xx dt bindings

Add the bindings for the bq256xx series of battery charging ICs.

Datasheets:
- https://www.ti.com/lit/ds/symlink/bq25600.pdf
- https://www.ti.com/lit/ds/symlink/bq25601.pdf
- https://www.ti.com/lit/ds/symlink/bq25600d.pdf
- https://www.ti.com/lit/ds/symlink/bq25601d.pdf
- https://www.ti.com/lit/ds/symlink/bq25611d.pdf
- https://www.ti.com/lit/ds/symlink/bq25618.pdf
- https://www.ti.com/lit/ds/symlink/bq25619.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>