dt-bindings: power: supply: max17042: Document max77849-battery

max77849 is a combined fuel-gauge, charger and MUIC device. Add it to
the bindings documentation.

Signed-off-by: Nikita Travkin <nikita@trvn.ru>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>