ARM: socfpga: Put stack at the end of SRAM

The global data are in the .data section, so there's no point in
reserving any space for it above stack. Put stack at the end of
SRAM.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <chin.liang.see@intel.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>