MIPS: Make the die_lock be raw

On preempt-rt this lock needs to be raw, so it does not get converted
to a sleeping spinlock. Trying to sleep in a panic is not really
desireable.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2636/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>