Add Allwinner A20 HDMI support.

The HDMI driver will attach a framebuffer device when a display is
connected. If the EDID can be read and contains a preferred mode, it
will be used. Otherwise the framebuffer will default to 800x600.

In addition, if the EDID contains a CEA-861 extension block and the
"basic audio" flag is set, audio playback at 48kHz 16-bit stereo is
enabled on the controller.

Reviewed by: andrew
Approved by: gonzo (mentor)
Differential Revision: https://reviews.freebsd.org/D5383