powerpc/83xx: Fix build error when CONFIG_PCI=n

As reported by lkp:

  arch/powerpc/platforms/83xx/km83xx.c:183:19: error: 'mpc83xx_setup_pci' undeclared here (not in a function)
     183 |  .discover_phbs = mpc83xx_setup_pci,
	 |                   ^~~~~~~~~~~~~~~~~
	 |                   mpc83xx_setup_arch

There is a stub defined for the CONFIG_PCI=n case, but now that
mpc83xx_setup_pci() is being assigned to discover_phbs the correct
empty value is NULL.

Fixes: 83f84041

 ("powerpc/83xx: Move PHB discovery")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210210130804.3190952-1-mpe@ellerman.id.au