ppc4xx: Export ECB and PLB emulation

Make these device models available outside ppc405_uc.c for reuse in
460EX emulation. They are left in their current place for now because
they are used mostly unchanged and I'm not sure these correctly model
the components in 440 SoCs (but they seem to be good enough). These
functions could be moved in a subsequent clean up series when this is
confirmed.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>