time: Provide xtime_update()

xtime_update() takes xtime_lock write locked and calls
do_timer(). Provided to replace the do_timer() calls in the
architecture code.

Signed-off-by: Torben Hohn <torbenh@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: johnstul@us.ibm.com
Cc: yong.zhang0@gmail.com
Cc: hch@infradead.org
LKML-Reference: <20110127145910.23248.21379.stgit@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>