ARM: Respect CONFIG_SPL_STACK define in lowlevel_init.S

The SPL and U-Boot proper may use different initial stack
locations, which are configured via CONFIG_SPL_STACK and
CONFIG_SYS_INIT_SP_ADDR defines. The lowlevel_init.S
code needs to handle this in the same way as crt0.S

Without this fix, setting the U-Boot stack location to some
place, which is not safely accessible by the SPL (such as
the DRAM), causes a very early SPL deadlock.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>