sparc: Switch do_timer() to xtime_update()

xtime_update() takes the xtime_lock itself.

pcic_clear_clock_irq() and clear_clock_irq do not need
to be protected by xtime_lock.

Signed-off-by: Torben Hohn <torbenh@gmx.de>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: johnstul@us.ibm.com
Cc: hch@infradead.org
Cc: yong.zhang0@gmail.com
LKML-Reference: <20110127150022.23248.80369.stgit@localhost>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>