posix-timers: Add cancel/arm callbacks

Add timer_try_to_cancel() and timer_arm() callbacks to kclock which allow
to make common_timer_set() usable by both hrtimer and alarmtimer based
clocks.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20170530211657.278022962@linutronix.de

kernel/time/posix-timers.h

@@ -21,6 +21,9 @@ struct k_clock {
 	void	(*timer_rearm)(struct k_itimer *timr);
 	int	(*timer_forward)(struct k_itimer *timr, ktime_t now);
 	ktime_t	(*timer_remaining)(struct k_itimer *timr, ktime_t now);
+	int	(*timer_try_to_cancel)(struct k_itimer *timr);
+	void	(*timer_arm)(struct k_itimer *timr, ktime_t expires,
+			     bool absolute, bool sigev_none);
 };
 
 extern const struct k_clock clock_posix_cpu;