Add basic support for MX31PDK board.

Add basic support to the MX31PDK development board, also known
as MX31 3DS or MX31 3-stack board (http://www.freescale.com/imx31pdk

).

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: sascha Hauer <s.hauer@pengutronix.de>