mfd: axp20x: Add ADC cells for AXP20X and AXP22X PMICs

This adds the AXP20X/AXP22x ADCs driver to the mfd cells of the AXP209,
AXP221 and AXP223 MFD.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>