armv8: ls1043ardb: Add the nand_bootcmd definition

nand_bootcmd doest not exist, it should be support
for nand auto boot up.

Signed-off-by: Wen He <wen.he_1@nxp.com>
Reviewed-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>