[POWERPC] Use lowmem_end_addr to limit lmb allocations on ppc32

Now that we have a proper variable that is the address of the top
of low memory we can use it to limit the lmb allocations.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>