xilinx: zynqmp: increase CONFIG_SYS_SPL_MALLOC_SIZE to 16MB

commit 03f1f78a ("spl: fit: Prefer a malloc()'d buffer for loading
images")' changed the way buffer allocation worked for SPL to a more
flexible method.

For xilinx zynqmp the 1MB buffer is not necessarily enough when dealing
with complex fit images (e.g. containing FPGA/TF-A/OP-TEE/U-Boot
proper), which can easily reach up to 10MB, so increase the default
CONFIG_SYS_SPL_MALLOC_SIZE size to 16MB to cover more advanced
scenarios.

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>