xtensa: Fix unnecessary setting of xtime

xtensa supports read_persisitent_clock(), so there is no reason for
the arch specific code to be setting xtime.  Setting xtime will be
done by the generic timekeeping code.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>