mips: convert to clocksource_register_hz/khz

This converts the mips clocksources to use clocksource_register_hz/khz

CC: Ralf Baechle <ralf@linux-mips.org>
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <johnstul@us.ibm.com>