board/t208xrdb: Add support of 2-stage NAND/SPI/SD boot

board/t208xrdb: Add support of 2-stage NAND/SPI/SD boot

Add support of 2-stage NAND/SPI/SD boot loader using SPL framework.
PBL initializes the internal CPC-SRAM and copy SPL(160K) to it,
SPL further initializes DDR using SPD and environment and copy
u-boot(768K) from SPI/SD/NAND to DDR, finally SPL transfers control
to u-boot.

Signed-off-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>