ARM: zynq: Change SYS_MALLOC_LEN in zynq_cse_nand_defconfig

nand_scan_tail() function allocates memory dynamically for
struct nand_buffers which needs ~21kbytes of memory. But the
memory alloted with CONFIG_SYS_MALLOC_LEN is 4k which is insufficient.
Increase CONFIG_SYS_MALLOC_LEN to 32Kbytes from which struct
nand_buffers uses ~21kbytes & remaining memory is used for other.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>