time: Consolidate the time accessor prototypes

Right now we have time related prototypes in 3 different header
files. Move it to a single timekeeping header file and move the core
internal stuff into a core private header.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

kernel/time/hrtimer.c

@@ -54,6 +54,8 @@
 
 #include <trace/events/timer.h>
 
+#include "timekeeping.h"
+
 /*
  * The timer bases:
  *