powerpc: Fix 32-bit compile: PPC_MEMSTART was undeclared

This defines PPC_MEMSTART as 0 because it is still used in a couple
of places in the 32-bit code.
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/page_32.h

@@ -3,6 +3,8 @@
 
 #define VM_DATA_DEFAULT_FLAGS	VM_DATA_DEFAULT_FLAGS32
 
+#define PPC_MEMSTART	0
+
 #ifndef __ASSEMBLY__
 /*
  * The basic type of a PTE - 64 bits for those CPUs with > 32 bit