arm: mmu: Add missing volatile for reading SCTLR register

Add 'volatile' qualifier to the asm statement in get_cr()
so that the statement is not optimized out by the compiler.

(http://comments.gmane.org/gmane.linux.linaro.toolchain/5163)

Without the 'volatile', get_cr() returns a wrong value which
prevents enabling the MMU  and later causes a PCIE VA access
failure.

Signed-off-by: Alison Wang <alison.wang@freescale.com>

arch/arm/include/asm/system.h

@@ -194,7 +194,7 @@ void save_boot_params_ret(void);
 static inline unsigned int get_cr(void)
 {
 	unsigned int val;
-	asm("mrc p15, 0, %0, c1, c0, 0	@ get CR" : "=r" (val) : : "cc");
+	asm volatile("mrc p15, 0, %0, c1, c0, 0	@ get CR" : "=r" (val) : : "cc");
 	return val;
 }