board/t104xrdb: Add support of NAND, SD, SPI boot for T104xRDB

Add support of 2 stage NAND, SD, SPI boot loader using SPL framework.
here, PBL initialise the internal SRAM and copy SPL(160KB). This further
initialise DDR using SPD and environment and copy u-boot(768 KB) from NAND to DDR.
Finally SPL transer control to u-boot.

Initialise/create followings required for SPL framework
      - Add spl.c which defines board_init_f, board_init_r
      - update tlb and ddr accordingly

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