mxc: add common debug board for 3-stack platforms

The debug board is little different for all mxc 3-stack(PDK)
platforms, it is possible here to add a common implementation to
support this board.

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>