arm: socfpga: Fix "improve raw MMC SPL boot"

This fixes commit d31e9c57 ,
which broke booting from SD card on all SoCFPGA boards. The
patch assumes the bootloader partition to be partition 3, at
the end of the SD card, which doesn't make any sense. U-Boot
assumes the bootloader partition is partition 1 or that the
bootloader image is at offset +1 MiB from the start of SD card.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Sylvain Lesne <lesne@alse-fr.com>