xilinx: zynqmp: Enable stack relocation to DDR

There is no space in OCM for SPL stack because the space in OCM is occupied
by TF-A. That's why move relocate stack to DDR to 0x18000000 address
and also enable SPL_SIZE_LIMIT not to be more then 0xfffea000 which is
default address for TFA.

It is good to summarize current DDR usage in SPL flow.
0-0x80000 is used for BSS
 (CONFIG_SPL_BSS_START_ADDR, CONFIG_SPL_BSS_MAX_SIZE)
0x100000 is used for DTB passing address
 (CONFIG_XILINX_OF_BOARD_DTB_ADDR)
0x17fffe70 - CONFIG_SPL_STACK_R_ADDR - is used for GD
0x18000000 is used for SPL stack
 (CONFIG_SPL_STACK_R_ADDR)
0x20000000-0x21000000 is used for SPL malloc area
 (CONFIG_SYS_SPL_MALLOC_START, CONFIG_SYS_SPL_MALLOC_SIZE)

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