arm: socfpga: Fix bootcounter located at the end of internal SRAM

Commit 768f23dc ("ARM: socfpga: Put stack at the end of SRAM") broke
those socfpga boards that keep the bootcounter at the end of the
internal SRAM as the bootcounter needs 8 bytes by default and thus the
very first SPL call to board_init_f_alloc_reserve overwrites the
bootcounter.

This patch allows to move the initial stack pointer down a bit by
checking if CONFIG_SYS_BOOTCOUNT_ADDR is located in the internal SRAM
area and then using this address as location for the start of the
stack pointer.

No new macros / defines are added by this approach.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>