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

Another typical thing to monitor via an ADC line is
the battery temperature.

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