Converts the scx200 clocksource to using clocksource_register_hz.

CC: Jim Cromie <jim.cromie@gmail.com>
Tested-by: Jim Cromie <jim.cromie@gmail.com>
Acked-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>