clocksource/drivers/timer-ti-dm: Prepare for using cpuidle

Let's add runtime_suspend and resume functions and atomic enabled
flag. This way we can use these when converting to use cpuidle
for saving and restoring device context.

And we need to maintain the driver state in the driver as documented
in "9. Autosuspend, or automatically-delayed suspends" in the
Documentation/power/runtime_pm.rst document related to using driver
private lock and races with runtime_suspend().

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200305082715.15861-3-lokeshvutla@ti.com

include/clocksource/timer-ti-dm.h

@@ -105,6 +105,7 @@ struct omap_dm_timer {
 	void __iomem	*pend;		/* write pending */
 	void __iomem	*func_base;	/* function register base */
 
+	atomic_t enabled;
 	unsigned long rate;
 	unsigned reserved:1;
 	unsigned posted:1;