dt-bindings: power: Convert ingenic,battery.txt to YAML

Convert the textual documentation of Device Tree bindings for the
Ingenic JZ47xx SoCs battery to YAML.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>