ARM: zynq: Reserve correct amount of non-DMA RAM

On Zynq, we haven't been reserving the correct amount of DMA-incapable
RAM to keep DMA away from it (per the Zynq TRM Section 4.1, it should be
the first 512k). In older kernels, this was masked by the
memblock_reserve call in arm_memblock_init(). Now, reserve the correct
amount excplicitly rather than relying on swapper_pg_dir, which is an
address and not a size anyway.

Fixes: 46f5b960 ("ARM: zynq: Reserve not DMAable space in front of the
kernel")

Signed-off-by: Kyle Roeschley <kyle.roeschley@ni.com>
Tested-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>