power: supply: ab8500_fg: Convert to IIO ADC

This switches the AB8500 fuel gauge driver to using
the standard IIO ADC channel lookup and conversion routines.

Acked-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>