ARM: zynq: Remove memory division by 2 for ECC case

For ECC case u-boot divided memory by 2 because one u-boot could be used
for both cases when ECC is off or on.
Remove this division and make sure that dts file contain the correct
memory size when ECC is enabled.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>