video: sii9234: dynamically register and unregister input device

The RCP input device should only be registered as long as MHL is
connected

Original author for tuna: Greg Hackmann <ghackmann@google.com>

Change-Id: I044b04e7d6ffe48c5b837981a8656ca2bb13fa01
Signed-off-by: Andreas B <skate4life@gmx.de>