documentation: media: Add documentation for new RGB and YUV bus formats

Add documentation for added Bus Formats to describe RGB and YUV formats used
as input to the Synopsys DesignWare HDMI TX Controller.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1491230558-10804-4-git-send-email-narmstrong@baylibre.com