[ARM] Don't read the CPU control reg back - it may be write only.

Some ARM CPUs don't allow CP15 CR1 control register to be read.
Therefore, to ensure that the value hits the control register on
Xscale, read back the CP15 CR0 ID register instead.

arch/arm/kernel/head.S

@@ -120,9 +120,9 @@ __turn_mmu_on:
 	orr	r0, r0, #2			@ ...........A.
 #endif
 	mcr	p15, 0, r0, c1, c0, 0		@ write control reg
-	mrc	p15, 0, r0, c1, c0, 0		@ read it back.
-	mov	r0, r0
-	mov	r0, r0
+	mrc	p15, 0, r3, c0, c0, 0		@ read id reg
+	mov	r3, r3
+	mov	r3, r3
 	mov	pc, lr
 
 /*