ARM: dts: sun6i: hummingbird-a31: Enable display output through VGA bridge

The Hummingbird A31 board has a VGA DAC which converts RGB output
from the LCD interface to VGA analog signals.

Add nodes for the VGA DAC, its power supply, and enable this part
of the display pipeline.

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