[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>
diff --git a/arch/powerpc/mm/pgtable_32.c b/arch/powerpc/mm/pgtable_32.c
index ac3390f..64c44bc 100644
--- a/arch/powerpc/mm/pgtable_32.c
+++ b/arch/powerpc/mm/pgtable_32.c
@@ -281,12 +281,13 @@
  */
 void __init mapin_ram(void)
 {
-	unsigned long v, p, s, f;
+	unsigned long v, s, f;
+	phys_addr_t p;
 	int ktext;
 
 	s = mmu_mapin_ram();
 	v = KERNELBASE + s;
-	p = PPC_MEMSTART + s;
+	p = memstart_addr + s;
 	for (; s < total_lowmem; s += PAGE_SIZE) {
 		ktext = ((char *) v >= _stext && (char *) v < etext);
 		f = ktext ?_PAGE_RAM_TEXT : _PAGE_RAM;