mfd: axp20x: Add AC power supply cells for AXP22X PMICs

The X-Powers AXP20X and AXP22X PMICs expose the status of AC power
supply.

This adds the AC power supply driver to the MFD cells of the AXP22X
PMICs.

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