microblaze: Fix stack protection behavior

microblaze: Fix stack protection behavior

When U-Boot starts stack protection can be already enabled that's why setup
the lowest possible SLR value which is address 0. And the highest possible
stack in front of U-Boot. That's why you should never load U-Boot to the
beginning of DDR. There must be some space reserved. Code is using this
location for early malloc space, early global data and stack.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/86b9748bad12142659804d6381bc6bbf20be44f1.1655299267.git.michal.simek@amd.com