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>
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 64843a8..1c2fe7d 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -54,6 +54,8 @@
 
 #include <trace/events/timer.h>
 
+#include "timekeeping.h"
+
 /*
  * The timer bases:
  *