mips/cpu: Expose play_dead()'s prototype definition

Include <asm/smp.h> to make sure play_dead() matches its prototype going
forward.

Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lkml.kernel.org/r/20230216184249.ogaqsaykottpxtcb@treble


Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>