Convert alpha to use clocksources instead of arch_gettimeoffset

Alpha has a tsc like rpcc counter that it uses to manage time.
This can be converted to an actual clocksource instead of utilizing
the arch_gettimeoffset method that is really only there for legacy
systems with no continuous counter.

Further cleanups could be made if alpha converted to the clockevent
model.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Richard Henderson <rth@twiddle.net>
Acked-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Tested-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: John Stultz <johnstul@us.ibm.com>