clockevents: Make clockevents_config() a global symbol

Make clockevents_config() into a global symbol to allow it to be used
by compiled-in clockevent drivers. This is needed by drivers that want
to update the timer frequency after registration time.

Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Simon Horman <horms@verge.net.au>
Cc: arnd@arndb.de
Cc: johnstul@us.ibm.com
Cc: rjw@sisk.pl
Cc: lethal@linux-sh.org
Cc: gregkh@linuxfoundation.org
Cc: olof@lixom.net
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20120509143934.27521.46553.sendpatchset@w520
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>