ARM: zynq: Fix location of stack and malloc areas

ARM: zynq: Fix location of stack and malloc areas

The patch
"board_init: Change the logic to setup malloc_base"
(sha1: 9ac4fc82071ce346e3885118242ff45d22f69b82)
breaks SPL for Zynq because it puts early alloc area on the stack which
caused that stack was decreased by CONFIG_SYS_MALLOC_F_LEN (0x400)
and there was not enough space for regular stack.

This patch changes memory layout to better utilize the last 64k OCM
block.
0xffff0000 - 0xfff1000 - Full malloc space
0xffff1000 - 0xffff300 - Stack location
0xfffff300 - CONFIG_SYS_MALLOC_F_LEN - Early malloc space
0xfffffd00 - sizeof(GD) - GD
0xfffffe00 - 0xffffffff - SoC specific boot code

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Moritz Fischer <moritz.fischer@ettus.com>