drm: bridge: dw-hdmi: Add Documentation on supported input formats

This patch adds a new DRM documentation entry and links to the input
format table added in the dw_hdmi header.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>