ARM: provide common method to clear bits in CPU control register

Several places open-code this manipulation, let's consolidate this.

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 7ea641b..ce727d4 100644
--- a/arch/arm/mm/mm.h
+++ b/arch/arm/mm/mm.h
@@ -2,6 +2,8 @@
 #include <linux/list.h>
 #include <linux/vmalloc.h>
 
+#include <asm/pgtable.h>
+
 /* the upper-most page table pointer */
 extern pmd_t *top_pmd;
 
@@ -93,3 +95,5 @@
 void __init bootmem_init(void);
 void arm_mm_memblock_reserve(void);
 void dma_contiguous_remap(void);
+
+unsigned long __clear_cr(unsigned long mask);