arm: sunxi: Add AXP20X_ADC

AXP20X_POWER depends on IIO. Even though it does not depend on AXP20X_ADC,
it is the new, preferred way of getting power supply configuration, it's
going to be enabled anyway.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>