Convert tcb_clksrc to use clocksource_register_hz.

CC: Nikolaus Voss <n.voss@weinmann.de>
CC: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Nikolaus Voss <n.voss@weinmann.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>