ARM: mvebu: handle unused DRAM banks with ECC enabled

dram_ecc_scrubbing() had code to skip unused DRAM banks but it would not
work because mvebu_sdram_bs() returns 0 and the code was subtracting 1
before checking the size. Remove the -1 from the bank size and the +1
from the total which will skip unused banks and still calculate the
correct size. Put the -1 where it is needed for scrubbing via the xor
engine.

Reported-by: Joshua Scott <joshua.scott@alliedtelesis.co.nz>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>