power: supply: generic-adc-battery: add DT support

This adds full DT support to the driver. Because of the previous
changes just adding a compatible value is enough.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>