[MIPS] AU1xxx mips_timer_interrupt() fixes

common/au1000/irq.c was missing a mips_timer_interrupt() prototype,
whereas in common/au1000/time.c the actual mips_timer_interrupt()
implementation was missing an irq_exit() invocation, causing a
preempt_count() leak.

Signed-off-by: Herbert Valerio Riedel <hvr@hvrlab.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>