power: supply: bq27xxx: adds specific support for bq27520-g4 revision.

This commit adds the BQ27520G4 chip definition to specifically match the
bq27520-G4 functionality as described in the datasheet.

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>