powerpc/85xx: Fix no previous prototype warning for mpc85xx_setup_pmc()

Fixes the following W=1 warning:
  arch/powerpc/platforms/85xx/mpc85xx_pm_ops.c:89:12: warning: no previous prototype for 'mpc85xx_setup_pmc'

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211124093254.1054750-1-mpe@ellerman.id.au