DRM: axi-hdmi: Add support for RGB output

Some versions of the core output a RGB instead of a YCbCr signal. This patch
adds support for them.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Add ZC706 + ADV7511 device tree

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>