dt-bindings: power: Convert power_supply text to yaml

Convert the power_supply.txt to power-supply.yaml.
This conversion entailed fixing up the binding to being yaml and dt
checker compliant.

Added a note in the power_supply.txt to reference the power-supply.yaml

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>