SMDK6400 can only boot U-Boot from NAND-flash. This patch adds a nand_spl
driver for it too. The board can also boot from the NOR flash, but due to
hardware limitations it can only address 64KiB on it, which is not enough
for U-Boot. Based on the original sources by Samsung for U-Boot 1.1.6.
Signed-off-by: NGuennadi Liakhovetski <lg@denx.de>