powerpc/kexec: kexec_sequence() is in misc_64.S

Correct reference to the location of the kexec_sequence() assembly helper.
There never was a kexec_stub.S in mainline.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/machine_kexec_64.c

@@ -312,7 +312,7 @@ static union thread_union kexec_stack __init_task_data =
  */
 struct paca_struct kexec_paca;
 
-/* Our assembly helper, in kexec_stub.S */
+/* Our assembly helper, in misc_64.S */
 extern void kexec_sequence(void *newstack, unsigned long start,
 			   void *image, void *control,
 			   void (*clear_all)(void)) __noreturn;