sparc/leon3: Initialize stack pointer

A lot of real world LEON3 systems are shipped with the GRMON boot
loader.  This boot loader initializes the stack pointer with the end of
RAM address.  The application can use this to detect the RAM size of a
particular board variant.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>
Reviewed-by: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
