time: Add y2038 safe update_persistent_clock64()

As part of addressing in-kernel y2038 issues, this patch adds
update_persistent_clock64() and replaces all the call sites of
update_persistent_clock() with this function. This is a __weak
implementation, which simply calls the existing y2038 unsafe
update_persistent_clock().

This allows architecture specific implementations to be
converted independently, and eventually y2038-unsafe
update_persistent_clock() can be removed after all its
architecture specific implementations have been converted to
update_persistent_clock64().

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1427945681-29972-4-git-send-email-john.stultz@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/rtc/systohc.c

@@ -11,7 +11,7 @@
  * rtc_set_ntp_time - Save NTP synchronized time to the RTC
  * @now: Current time of day
  *
- * Replacement for the NTP platform function update_persistent_clock
+ * Replacement for the NTP platform function update_persistent_clock64
  * that stores time for later retrieval by rtc_hctosys.
  *
  * Returns 0 on successful RTC update, -ENODEV if a RTC update is not