ARM: zynq: Extend malloc size

SPL DM MMC FAT requires more malloc space(3k fat buffers + dm)
that it is available now. Extend SPL malloc space.

Current OCM layout:
0xffff0000 - 0xfff2000 - Full malloc space
0xffff2000 - 0xffff300 - Stack location
0xfffff300 - CONFIG_SYS_MALLOC_F_LEN - Early malloc space
0xfffffd00 - sizeof(GD) - GD
0xfffffe00 - 0xffffffff - SoC specific boot code

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