power: supply: bq27xxx: adds device tree binding documentation.

The bq27xxx binding is a standard i2c style binding, however the
deprecated compatible fields and different revisions warrant its own
documentation.

Signed-off-by: Chris Lapa <chris@lapa.com.au>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
Reviewed-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>