arm64: zynqmp: Reduce malloc memory for mini QSPI configuration

Mini U-boot runs on lower foot print of 256KB OCM. Hence 8K memory
for malloc may not be required. Reduce it by 1.5K.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>