ARM: pxa: only save/restore registers when pm functions are defined

Reported-and-tested-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>

arch/arm/mach-pxa/pm.c

@@ -33,7 +33,7 @@ int pxa_pm_enter(suspend_state_t state)
 #endif
 
 	/* skip registers saving for standby */
-	if (state != PM_SUSPEND_STANDBY) {
+	if (state != PM_SUSPEND_STANDBY && pxa_cpu_pm_fns->save) {
 		pxa_cpu_pm_fns->save(sleep_save);
 		/* before sleeping, calculate and save a checksum */
 		for (i = 0; i < pxa_cpu_pm_fns->save_count - 1; i++)
@@ -44,7 +44,7 @@ int pxa_pm_enter(suspend_state_t state)
 	pxa_cpu_pm_fns->enter(state);
 	cpu_init();
 
-	if (state != PM_SUSPEND_STANDBY) {
+	if (state != PM_SUSPEND_STANDBY && pxa_cpu_pm_fns->restore) {
 		/* after sleeping, validate the checksum */
 		for (i = 0; i < pxa_cpu_pm_fns->save_count - 1; i++)
 			checksum += sleep_save[i];