powerpc/mm: Fix build error in setup_initial_memory_limit

arch/powerpc/mm/tlb_nohash.c: In function 'setup_initial_memory_limit':
arch/powerpc/mm/tlb_nohash.c:588:29: error: 'ppc64_memblock_base' undeclared (first use in this function)
arch/powerpc/mm/tlb_nohash.c:588:29: note: each undeclared identifier is reported only once for each function it appears in

Due to a copy/paste typo with the following commit:

	commit cd3db0c4
	Author: Benjamin Herrenschmidt <benh@kernel.crashing.org>
	Date:   Tue Jul 6 15:39:02 2010 -0700

	    memblock: Remove rmo_size, burry it in arch/powerpc where it belongs

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>