powerpc/t4rdb: Add SD boot support for T4240RDB board

This patch adds SD boot support for T4240RDB board. SPL
framework is used. PBL initializes the internal RAM and
copies SPL to it. Then SPL initializes DDR using SPD and
copies u-boot from SD card to DDR, finally SPL transfers
control to u-boot.

Signed-off-by: Chunhe Lan <Chunhe.Lan@freescale.com>
[York Sun: Fix T4240RDB_SDCARD_defcofig]
Reviewed-by: York Sun <yorksun@freescale.com>