mfd: Guard adp5520 PM methods with CONFIG_PM_SLEEP

This fixes below build warnings:
  CC      drivers/mfd/adp5520.o
drivers/mfd/adp5520.c:324:12: warning: 'adp5520_suspend' defined but not used [-Wunused-function]
drivers/mfd/adp5520.c:333:12: warning: 'adp5520_resume' defined but not used [-Wunused-function]
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/adp5520.c

@@ -320,7 +320,7 @@ static int __devexit adp5520_remove(struct i2c_client *client)
 	return 0;
 }
 
-#ifdef CONFIG_PM
+#ifdef CONFIG_PM_SLEEP
 static int adp5520_suspend(struct device *dev)
 {
 	struct i2c_client *client = to_i2c_client(dev);