AArch32: Fix the stack alignment issue

The AArch32 Procedure call Standard mandates that the stack must be aligned
to 8 byte boundary at external interfaces. This patch does the required
changes.

This problem was detected when a crash was encountered in
`psci_print_power_domain_map()` while printing 64 bit values. Aligning
the stack to 8 byte boundary resolved the problem.

Fixes ARM-Software/tf-issues#437

Change-Id: I517bd8203601bb88e9311bd36d477fb7b3efb292
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
diff --git a/lib/cpus/aarch32/cpu_helpers.S b/lib/cpus/aarch32/cpu_helpers.S
index 042ffbd..a4dfe5f 100644
--- a/lib/cpus/aarch32/cpu_helpers.S
+++ b/lib/cpus/aarch32/cpu_helpers.S
@@ -76,9 +76,10 @@
 	 */
 	.globl	prepare_core_pwr_dwn
 func prepare_core_pwr_dwn
-	push	{lr}
+	/* r12 is pushed to meet the 8 byte stack alignment requirement */
+	push	{r12, lr}
 	bl	_cpu_data
-	pop	{lr}
+	pop	{r12, lr}
 
 	ldr	r1, [r0, #CPU_DATA_CPU_OPS_PTR]
 #if ASM_ASSERTION
@@ -98,9 +99,10 @@
 	 */
 	.globl	prepare_cluster_pwr_dwn
 func prepare_cluster_pwr_dwn
-	push	{lr}
+	/* r12 is pushed to meet the 8 byte stack alignment requirement */
+	push	{r12, lr}
 	bl	_cpu_data
-	pop	{lr}
+	pop	{r12, lr}
 
 	ldr	r1, [r0, #CPU_DATA_CPU_OPS_PTR]
 #if ASM_ASSERTION