[MIPS] Protect more of timer_interrupt() by xtime_lock.

    
From Dave Johnson <djohnson+linuxmips@sw.starentnetworks.com>:

* do_timer() expects the arch-specific handler to take the lock as it
  modifies jiffies[_64] and xtime.
* writing timerhi/lo in timer_interrupt() will mess up
  fixed_rate_gettimeoffset() which reads timerhi/lo.
    
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>