dt-bindings: power: supply: adc-battery: add binding

Add binding for a battery that is only monitored via ADC
channels and simple status GPIOs.

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