ARM: dtsi: axp22x: add battery power supply subnode

The X-Powers AXP22X PMIC exposes battery supply various data such as
the battery status (charging, discharging, full, dead), current max
limit, current current, battery capacity (in percentage), voltage max
limit, current voltage, and battery capacity (in Ah).

This adds the battery power supply subnode for AXP22X PMIC.

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: Maxime Ripard <maxime.ripard@free-electrons.com>