ARM: dts: am57xx: compulab-sb-som: add HDMI connector

Add HDMI connector node without a valid input endpoint.

CompuLab SB-SOM is a carrier board, hence the endpoint
should be added in the board DT with a valid HDMI output.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>