arm/mach-clps711x: Switch do_timer() to xtime_update()

do_timer() requires holding the xtime_lock, which this
code did not do. Use the safe version xtime_update()

Signed-off-by: Torben Hohn <torbenh@gmx.de>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: johnstul@us.ibm.com
Cc: hch@infradead.org
Cc: yong.zhang0@gmail.com
LKML-Reference: <20110127145926.23248.56369.stgit@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>