dt-bindings: document gpio-charger bindings

This documents the binding for the gpio-charger power-supply.

Signed-off-by: Heiko Stuebner <heiko.stuebner@bq.com>
power: gpio-charger: add device tree support

Add the ability to parse gpio-charger data from a devicetree node.

Signed-off-by: Heiko Stuebner <heiko.stuebner@bq.com>
power: gpio-charger: do not use gpio value directly

Some gpio implementations return interesting values for gpio_get_value when
the value is not 0 - as seen on a imx6sl board. Therefore do not use the
value returned from gpio_get_value directly but simply check for 0 or not 0.

Signed-off-by: Heiko Stuebner <heiko.stuebner@bq.com>