powerpc/mm: Use PFN_PHYS() in devmem_is_allowed()

This function can run on systems where physical addresses don't
fit in unsigned long, so make sure to use the macro that contains the
proper cast.
Signed-off-by: Scott Wood <scottwood@freescale.com>

arch/powerpc/mm/mem.c

@@ -560,7 +560,7 @@ subsys_initcall(add_system_ram_resources);
  */
 int devmem_is_allowed(unsigned long pfn)
 {
-	if (iomem_is_exclusive(pfn << PAGE_SHIFT))
+	if (iomem_is_exclusive(PFN_PHYS(pfn)))
 		return 0;
 	if (!page_is_ram(pfn))
 		return 1;