dt-bindings: power: supply: add AXP20X/AXP22X AC power supply

The X-Powers AXP20X and AXP22X PMICs have an AC entry to supply power to
the board. They have a few registers dedicated to the status of the AC
power supply.

This adds the DT binding documentation for the AC power supply for
AXP20X and AXP22X PMICs.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>