[ARM] Fix nommu build

Fix warnings and errors in arch/arm/mm for nommu build.
Remove commented out function prototype in pgtable-nommu.h

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/mm/mm.h b/arch/arm/mm/mm.h
index bb2bc9a..a44e309 100644
--- a/arch/arm/mm/mm.h
+++ b/arch/arm/mm/mm.h
@@ -1,4 +1,7 @@
 /* the upper-most page table pointer */
+
+#ifdef CONFIG_MMU
+
 extern pmd_t *top_pmd;
 
 #define TOP_PTE(x)	pte_offset_kernel(top_pmd, x)
@@ -13,6 +16,8 @@
 	return pmd_off(pgd_offset_k(virt), virt);
 }
 
+#endif
+
 struct map_desc;
 struct meminfo;
 struct pglist_data;