sunxi: Hummingbird_A31_defconfig: Enable VGA output through external DAC

The Hummingbird A31 uses an external DAC connected to the LCD0 outputs
for the on board VGA output. The DAC has a power control that's toggled
by GPIO.

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