powerpc/t1023rdb: eMMC boot without external SD card

eMMC has no CD and WP pins, it needs to add board-specific
board_mmc_getcd() and board_mmc_getwp() in SPL to support
eMMC boot without external SD card inserted.

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