MIPS: Add IRQF_TIMER flag for timer interrupts

Along the lines of d6c585a4

, add IRQF_TIMER
flag for all timer interrupts  This ensures that timer interrupts won't be
disabled on suspend and not threaded for PREEMPT_RT.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>