[ARM] Fix timer damage from d3d74453c34f8fd87674a8cf5b8a327c68f22e99

Move the xtime write mode seqlock into timer_tick(), so it only
surrounds the call to do_timer().

This avoids a deadlock in update_process_times() ...
hrtimer_get_softirq_time() which tries to get a read mode seqlock
on xtime, thereby preventing booting.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>