sunxi: video: Add hdmi support

So far we've been programming the hdmi-encoder to send out dvi data over the
hdmi connector. This works well for most devices, including hdmi devices, but
not all devices accept dvi data on a hdmi input.

Add support for sending proper hdmi data over the hdmi output found on most
sunxi boards. This can be turned on by adding monitor=hdmi as option to the
video-mode env. variable.

A follow up patch will determine whether to send dvi or hdmi automatically when
EDID is used.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Anatolij Gustschin <agust@denx.de>