avr32: Convert to clocksource_register_hz

This converts the avr32 clocksource to use clocksource_register_hz.

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

CC: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <johnstul@us.ibm.com>