powerpc: Merge lmb.c and make MM initialization use it.

This also creates merged versions of do_init_bootmem, paging_init
and mem_init and moves them to arch/powerpc/mm/mem.c.  It gets rid
of the mem_pieces stuff.

I made memory_limit a parameter to lmb_enforce_memory_limit rather
than a global referenced by that function.  This will require some
small changes to ppc64 if we want to continue building ARCH=ppc64
using the merged lmb.c.

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/mm/mmu_decl.h

@@ -36,6 +36,8 @@ extern unsigned long ioremap_base;
 extern unsigned long ioremap_bot;
 extern unsigned int rtas_data, rtas_size;
 
+extern unsigned long __max_low_memory;
+extern unsigned long __initial_memory_limit;
 extern unsigned long total_memory;
 extern unsigned long total_lowmem;
 extern int mem_init_done;