[POWERPC] Remove and replace uses of PPC_MEMSTART with memstart_addr

A number of users of PPC_MEMSTART (40x, ppc_mmu_32) can just always
use 0 as we don't support booting these kernels at non-zero physical
addresses since their exception vectors must be at 0 (or 0xfffx_xxxx).

For the sub-arches that support relocatable interrupt vectors
(book-e), it's reasonable to have memory start at a non-zero physical
address.  For those cases use the variable memstart_addr instead of
the #define PPC_MEMSTART since the only uses of PPC_MEMSTART are for
initialization and in the future we can set memstart_addr at runtime
to have a relocatable kernel.

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

arch/powerpc/mm/mmu_decl.h

@@ -51,6 +51,7 @@ extern unsigned long __max_low_memory;
 extern unsigned long __initial_memory_limit;
 extern unsigned long total_memory;
 extern unsigned long total_lowmem;
+extern phys_addr_t memstart_addr;
 
 /* ...and now those things that may be slightly different between processor
  * architectures.  -- Dan