ARM: mxs: request clock for timer

When mxs_timer_init() does not have a timer_clk passed in, it should
try to request clock from clkdev system.
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

arch/arm/mach-mxs/timer.c

@@ -20,6 +20,7 @@
  * MA 02110-1301, USA.
  */
 
+#include <linux/err.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>
 #include <linux/clockchips.h>
@@ -245,6 +246,14 @@ static int __init mxs_clocksource_init(struct clk *timer_clk)
 
 void __init mxs_timer_init(struct clk *timer_clk, int irq)
 {
+	if (!timer_clk) {
+		timer_clk = clk_get_sys("timrot", NULL);
+		if (IS_ERR(timer_clk)) {
+			pr_err("%s: failed to get clk\n", __func__);
+			return;
+		}
+	}
+
 	clk_prepare_enable(timer_clk);
 
 	/*