clockevents: export clockevents_config_and_register for module use

clockevents_config_and_register is a handy helper for clockevent
drivers, some of which might support module build, so export the symbol.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Olof Johansson <olof@lixom.net>