timecounter: provide a helper function to shift the time.

Some PTP Hardware Clock drivers use a struct timecounter to represent
their clock. To adjust the time by a given offset, these drivers all
perform a two step read/write of their timecounter. However, it is
better and simpler just to adjust the offset in one step. This patch
introduces a little routine to help drivers implement the adjtime
method.
Suggested-by: NJanusz Użycki <j.uzycki@elproma.com.pl>
Signed-off-by: NRichard Cochran <richardcochran@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/timecounter.h

@@ -78,6 +78,15 @@ static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc,
 	return ret;
 }
 
+/**
+ * timecounter_adjtime - Shifts the time of the clock.
+ * @delta:	Desired change in nanoseconds.
+ */
+static inline void timecounter_adjtime(struct timecounter *tc, s64 delta)
+{
+	tc->nsec += delta;
+}
+
 /**
  * timecounter_init - initialize a time counter
  * @tc:			Pointer to time counter which is to be initialized/reset