mc3: Add onboard and backplane HDMI support

At the moment there is a problem with the onboard HDMI hot plug detection (HPD) in combination with the eARC. The HPD only works correctly when the EARC_N_HPD signal is disconnected. Simply enabling or disabling the xcvr (eARC) controller does not result in a valid HPD. HDMI works when the eARC is not connected.