ARM: sun6i: a31s: primo81: Enable battery power supply

The MSI Primo81 tablet has a 3500 mAh 3.7V LiPo battery.

Enable the PMIC's battery power supply so the battery can be monitored.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>