ia64: convert to clocksource_register_hz/khz

This converts the ia64 clocksources to use clocksource_register_hz/khz

CC: Tony Luck <tony.luck@intel.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Tony Luck <tony.luck@intel.com> [clocksource_itc path]
Signed-off-by: John Stultz <johnstul@us.ibm.com>