microblaze: convert to clocksource_register_hz/khz

This converts the microblaze clocksources to use clocksource_register_hz/khz

CC: Michal Simek <monstr@monstr.eu>
CC: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: John Stultz <johnstul@us.ibm.com>