SPL: add support to boot from a partition type

the socfpga bootrom supports mmc booting from either a raw image
starting at 0x0, or from a partition of type 0xa2.  This patch
adds support for locating the boot image in the first type 0xa2
partition found.

Assigned a partition number of -1 will cause a search for a
partition of type CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION_TYPE
and use it to find the u-boot image

Signed-off-by: Dalon Westergreen <dwesterg@gmail.com>