powerpc/85xx: Make mpc85xx_smp_kexec_cpu_down() static

To fix the W=1 warning:
  arch/powerpc/platforms/85xx/smp.c:369:6: error: no previous prototype for ‘mpc85xx_smp_kexec_cpu_down’
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211124093254.1054750-2-mpe@ellerman.id.au

arch/powerpc/platforms/85xx/smp.c

@@ -366,7 +366,7 @@ struct smp_ops_t smp_85xx_ops = {
 #ifdef CONFIG_PPC32
 atomic_t kexec_down_cpus = ATOMIC_INIT(0);
 
-void mpc85xx_smp_kexec_cpu_down(int crash_shutdown, int secondary)
+static void mpc85xx_smp_kexec_cpu_down(int crash_shutdown, int secondary)
 {
 	local_irq_disable();
 
@@ -384,7 +384,7 @@ static void mpc85xx_smp_kexec_down(void *arg)
 		ppc_md.kexec_cpu_down(0,1);
 }
 #else
-void mpc85xx_smp_kexec_cpu_down(int crash_shutdown, int secondary)
+static void mpc85xx_smp_kexec_cpu_down(int crash_shutdown, int secondary)
 {
 	int cpu = smp_processor_id();
 	int sibling = cpu_last_thread_sibling(cpu);