powerpc/85xx: Fix declaration made after definition

Currently the clang build of corenet64_smp_defconfig fails with:

  arch/powerpc/platforms/85xx/corenet_generic.c:210:1: error:
  attribute declaration must precede definition
  machine_arch_initcall(corenet_generic, corenet_gen_publish_devices);

Fix it by moving the initcall definition prior to the machine
definition, and directly below the function it calls, which is the
usual style anyway.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201023020838.3274226-1-mpe@ellerman.id.au