MN10300: Use clocksource_register_hz()

clocksource_register_hz() calculates the shift/mult pair for the
clocksource. Remove the mn10300 duplicate implementation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <johnstul@us.ibm.com>
Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com>
Cc: Mark Salter <msalter@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>