arm: ls102xa: Add NAND boot support for LS1021AQDS board

This patch adds NAND boot support for LS1021AQDS board. SPL
framework is used. PBL initialize the internal RAM and copy
SPL to it, then SPL initialize DDR using SPD and copy u-boot
from NAND flash to DDR, finally SPL transfer control to u-boot.

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Signed-off-by: Alison Wang <alison.wang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>