ARM: dtsi: sun8i: a711: enable battery power supply subnode

The TBS A711 has an AXP813 PMIC and a soldered battery, thus, we enable
the battery power supply subnode in its Device Tree.

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>