MX31: Add NAND SPL for i.MX31.

This patch adds the NAND SPL framework needed to boot i.MX31 boards
from NAND.

It has been tested on a i.MX31 PDK board with large page NAND. Small
page NANDs should work as well, but this has not been tested.

Note: The i.MX31 NFC uses a non-standard layout for large page NANDs,
whether this is compatible with a particular setup depends on how
the NAND device is programmed by the flash programmer (e.g. JTAG
debugger).

The patch is based on the work by Maxim Artamonov.

Signed-off-by: Maxim Artamonov <scn1874@yandex.ru>
Signed-off-by: Magnus Lilja <lilja.magnus@gmail.com>