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/82xx/pq2.h

@@ -1,7 +1,7 @@
 #ifndef _PQ2_H
 #define _PQ2_H
 
-void pq2_restart(char *cmd);
+void __noreturn pq2_restart(char *cmd);
 
 #ifdef CONFIG_PCI
 int pq2ads_pci_init_irq(void);