clocksource: xtensa: Convert to clocksource_register_hz/khz

This converts the xtensa clocksource to use clocksource_register_hz/khz

This is untested, so any assistance in testing would be appreciated!

CC: Chris Zankel <chris@zankel.net>
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <johnstul@us.ibm.com>