arm: ls102xa: Add SD boot support for LS1021ATWR board

This patch adds SD boot support for LS1021ATWR 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 SD card to DDR, finally SPL transfer control to u-boot.

Signed-off-by: Chen Lu <chen.lu@freescale.com>
Signed-off-by: Alison Wang <alison.wang@freescale.com>
Signed-off-by: Jason Jin <jason.jin@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>