clocksource: em_sti: Add DT support

Update the em-sti driver to support DT.

Signed-off-by: Magnus Damm <damm@opensource.se>
Cc: arnd@arndb.de
Cc: horms@verge.net.au
Cc: johnstul@us.ibm.com
Cc: rjw@sisk.pl
Cc: lethal@linux-sh.org
Cc: gregkh@linuxfoundation.org
Cc: olof@lixom.net
Link: http://lkml.kernel.org/r/20120509143950.27521.7949.sendpatchset@w520


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>