arm: Switch from do_timer() to xtime_update()

xtime_update takes the xtime_lock itself.

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: <20110127145920.23248.75541.stgit@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>