sparc: Convert sparc to use read/update_persistent_clock

This patch converts the sparc architecture to use the generic
read_persistent_clock and update_persistent_clock interfaces, reducing
the amount of arch specific code we have to maintain, and allowing for
further cleanups in the future.

[ davem: compile fix: Here's a version that compiles, you have to get
  rid of the now unused variably last_rtc_update since we build with
  -Werror ]
 
Signed-off-by: John Stultz <johnstul@us.ibm.com>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: sparclinux@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>