arm: mvebu: Add complete SDRAM ECC scrubbing

This patch introduces the SDRAM scrubbing for ECC enabled board
to fill/initialize the ECC bytes. This is done via the XOR engine
to speed up the process. The scrubbing is a 2-stage process:

1) SPL scrubs the area 0 - 0x100.0000 (16MiB) for the main U-Boot
2) U-Boot scrubs the remaining SDRAM area(s)

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>