microblaze: Use klimit instead of _end for memory init

For noMMU system when you use larger rootfs image
there is problem with using _end label because
we increase klimit but in memory initialization
we use still _end which is wrong. Larger mtd rootfs
was rewritten by init_bootmem_node.

MMU kernel use static initialization where klimit
is setup to _end. There is no any other hanling
with klimit.

Signed-off-by: Michal Simek <monstr@monstr.eu>