arm: socfpga: improve raw MMC SPL boot

Before this patch, when booting from MMC (no filesystem), the SPL
loaded U-Boot from a fixed offset.
It will now load U-Boot from an offset of 256kB (which is 4 times the
padded SPL image) in the third partition.

This behaviour is similar to what the vendor SPL (based on
U-Boot 2013.01) does, and allows to directly 'dd' the
u-boot-with-spl.sfp file to the A2 partition.

Signed-off-by: Sylvain Lesne <lesne@alse-fr.com>