ARM: dtsi: axp81x: add battery power supply subnode

The X-Powers AXP81X 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
and min limits, current voltage, and battery capacity (in Ah).

This adds the battery power supply subnode for AXP81X PMIC.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>