microblaze: Fix early stack allocation

microblaze: Fix early stack allocation

CONFIG_SYS_INIT_SP_OFFSET macro place stack to TEXT_BASE - SYS_MALLOC_F_LEN
but there is no reason to do it now because board_init_f_alloc_reserve()
returns exact location where stack should be. That's why stack location is
calculated at run time and there is no need to hardcode it via macro. This
change will help with placing U-Boot to any address.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/e9aee69646e022fd8a96cbee2d2a07ab81fb6e05.1655299267.git.michal.simek@amd.com