Convert clocksource_dbx500_prcmu to use clocksource_register_hz.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mattias Wallin <mattias.wallin@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>