dt-bindings: power: Add BQ27Z561 compatible

Add the Texas Instruments bq27z561 battery monitor to the bq27xxx
binding.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>