ARM: socfpga: Increase Malloc pool size to support FAT filesystem in SPL

Increasing Malloc pool size up to 0x15000 is required to support FAT in SPL
. The result of calculation is come from default max cluster(0x10000) +
others(0x2000) + additional memory for headroom(0x3000).

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>