dt-bindings: power: supply: add AXP20X/AXP22X battery DT binding

The X-Powers AXP20X and AXP22X PMICs can have a battery as power supply.

This patch adds the DT binding documentation for the battery power
supply which gets various data from the PMIC, such as the battery status
(charging, discharging, full, dead), current max limit, current current,
battery capacity (in percentage), voltage max and min limits, current
voltage and battery capacity (in Ah).

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>