powerpc: Fix 8xx build failure

The 'powerpc ioremap_prot' broke 8xx builds:

include2/asm/pgtable-ppc32.h:555: error: '_PAGE_WRITETHRU' undeclared (first use in this function)
include2/asm/pgtable-ppc32.h:555: error: (Each undeclared identifier is reported only once
include2/asm/pgtable-ppc32.h:555: error: for each function it appears in.)
Signed-off-by: NKumar Gala <galak@kernel.crashing.org>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

include/asm-powerpc/pgtable-ppc32.h

@@ -401,6 +401,9 @@ extern int icache_44x_need_flush;
 #ifndef _PAGE_COHERENT
 #define _PAGE_COHERENT	0
 #endif
+#ifndef _PAGE_WRITETHRU
+#define _PAGE_WRITETHRU	0
+#endif
 #ifndef _PMD_PRESENT_MASK
 #define _PMD_PRESENT_MASK	_PMD_PRESENT
 #endif