SPL: ONENAND: Support SPL to boot u-boot from OneNAND.

This patch will allow use SPL to boot an u-boot from the OneNAND.

Tested with IGEPv2 board with a OneNAND from Numonyx

Signed-off-by: Enric Balletbo i Serra <eballetbo@iseebcn.com>
[trini: Add <spl.h> hunk to fix warning]
Signed-off-by: Tom Rini <trini@ti.com>