NAND: Add ECC support to NAND booting support in nand_spl/nand_boot.c

The U-Boot NAND booting support is now extended to support ECC
upon loading of the NAND U-Boot image.

Tested on AMCC Sequoia (440EPx) and Bamboo (440EP).

Signed-off-by: Stefan Roese <sr@denx.de>