ARM: dts: Gumstix DuoVero: Bind vdac regulator to hdmi node

The HDMI node does not have a power supply attached. As a result its
power regulator, VDAC, shuts off on boot and screen loses signal.
This attaches VDAC (vdda_hdmi_dac) to HDMI's vdda-supply.

Signed-off-by: Adam YH Lee <adam.yh.lee@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>