sunxi: Enable AXP818 SW for Sinovoip BPI M3

The SW output of the PMIC supplies the ethernet PHY with power.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>