dt-bindings: power: supply: ab8500: Convert to DT schema format

Convert the binding to DT schema format.

Note: The battery node does not have a compatible value and needs
to be described from the binding file for the PMIC. That has not
yet been converted, so I kept the information in plaintext for now.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>