arm64: zynqmp: Decrease MALLOC size for SPL

Decrease malloc size to 1MB from 256MB. Huge malloc
space is adding huge delay in mem_malloc_init() because
we are enabling CONFIG_SYS_MALLOC_CLEAR_ON_INIT
which clear the whole malloc space.

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