The Cubietruck has an AXP209 PMIC with battery connector.

This enables the battery power supply subnode.
Signed-off-by: Alexander Syring <alex@asyring.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
[wens@csie.org: Correct subject prefix order]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>