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