powerpc/cpu: Mark start_secondary_resume() __noreturn

start_secondary_resume() doesn't return.  Annotate it as such.  By
extension this also makes arch_cpu_idle_dead() noreturn.

Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/b6b2141f832d8cd8ade65f190d04b011cda5f9bb.1676358308.git.jpoimboe@kernel.orgSigned-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

arch/powerpc/include/asm/smp.h

@@ -67,7 +67,7 @@ void start_secondary(void *unused);
 extern int smp_send_nmi_ipi(int cpu, void (*fn)(struct pt_regs *), u64 delay_us);
 extern int smp_send_safe_nmi_ipi(int cpu, void (*fn)(struct pt_regs *), u64 delay_us);
 extern void smp_send_debugger_break(void);
-extern void start_secondary_resume(void);
+extern void __noreturn start_secondary_resume(void);
 extern void smp_generic_give_timebase(void);
 extern void smp_generic_take_timebase(void);