loongarch/cpu: Mark play_dead() __noreturn

play_dead() doesn't return.  Annotate it as such.  By extension this
also makes arch_cpu_idle_dead() noreturn.
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/4da55acfdec8a9132c4e21ffb7edb1f846841193.1676358308.git.jpoimboe@kernel.orgSigned-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

arch/loongarch/include/asm/smp.h

@@ -99,7 +99,7 @@ static inline void __cpu_die(unsigned int cpu)
 	loongson_cpu_die(cpu);
 }
 
-extern void play_dead(void);
+extern void __noreturn play_dead(void);
 #endif
 
 #endif /* __ASM_SMP_H */