Add support for Freescale's i.MX31 PDK board (a.k.a. 3 stack board).

This patch assumes that some other program performs the actual
NAND boot.

Signed-off-by: Magnus Lilja <lilja.magnus@gmail.com>
Acked-by: Fabio Estevam <fabioestevam@yahoo.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>