powerpc: Make ppc_md.{halt, restart} __noreturn

powernv marks it's halt and restart calls as __noreturn. However,
ppc_md does not have this annotation. Add the annotation to ppc_md,
and then to every halt/restart function that is missing it.

Additionally, I have verified that all of these functions do not
return. Occasionally I have added a spin loop to be sure.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/amigaone/setup.c

@@ -123,7 +123,7 @@ static int __init request_isa_regions(void)
 }
 machine_device_initcall(amigaone, request_isa_regions);
 
-void amigaone_restart(char *cmd)
+void __noreturn amigaone_restart(char *cmd)
 {
 	local_irq_disable();