clocksource/drivers/arm_global_timer: Fix suspend resume

Now the System stall is observed on TI AM437x based board (am437x-gp-evm)
during resuming from System suspend when ARM Global timer is selected as
clocksource device (CPUIdle not enabled) - SysRq are working, but nothing
else.

The reason of stall is that ARM Global timer loses its contexts during
System suspend:
   GT_CONTROL.TIMER_ENABLE = 0 (unbanked)
   GT_COUNTERx = 0

Hence, update ARM Global timer driver to reflect above behaviour
- re-enable ARM Global timer on resume (GT_CONTROL.TIMER_ENABLE = 1)
  if not enabled.

CC: Arnd Bergmann <arnd@arndb.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/clocksource/arm_global_timer.c

@@ -195,12 +195,23 @@ static cycle_t gt_clocksource_read(struct clocksource *cs)
 	return gt_counter_read();
 }
 
+static void gt_resume(struct clocksource *cs)
+{
+	unsigned long ctrl;
+
+	ctrl = readl(gt_base + GT_CONTROL);
+	if (!(ctrl & GT_CONTROL_TIMER_ENABLE))
+		/* re-enable timer on resume */
+		writel(GT_CONTROL_TIMER_ENABLE, gt_base + GT_CONTROL);
+}
+
 static struct clocksource gt_clocksource = {
 	.name	= "arm_global_timer",
 	.rating	= 300,
 	.read	= gt_clocksource_read,
 	.mask	= CLOCKSOURCE_MASK(64),
 	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
+	.resume = gt_resume,
 };
 
 #ifdef CONFIG_CLKSRC_ARM_GLOBAL_TIMER_SCHED_CLOCK