sprd_serial: compile sprd_suspend and sprd_resume conditionally

Building sprd_serial.o when CONFIG_PM_SLEEP is not defined triggers
these warnings:
    drivers/tty/serial/sprd_serial.c:755:12: warning: ‘sprd_suspend’ defined but not used [-Wunused-function]
     static int sprd_suspend(struct device *dev)
                ^
    drivers/tty/serial/sprd_serial.c:764:12: warning: ‘sprd_resume’ defined but not used [-Wunused-function]
     static int sprd_resume(struct device *dev)
                ^

Let's compile these functions only when CONFIG_PM_SLEEP is defined.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sprd_serial.c

@@ -752,6 +752,7 @@ static int sprd_probe(struct platform_device *pdev)
 	return ret;
 }
 
+#ifdef CONFIG_PM_SLEEP
 static int sprd_suspend(struct device *dev)
 {
 	struct sprd_uart_port *sup = dev_get_drvdata(dev);
@@ -769,6 +770,7 @@ static int sprd_resume(struct device *dev)
 
 	return 0;
 }
+#endif
 
 static SIMPLE_DEV_PM_OPS(sprd_pm_ops, sprd_suspend, sprd_resume);