dpaa2: Don't return simplebus devinfo for a non-simplebus child

[mmel]

Summary

dpaa2_mc_simplebus_get_devinfo() must not treat the MC resource-container
device as a simplebus child. Return NULL when child is sc->rcdev instead
of forwarding OFW_BUS_GET_DEVINFO() for that device.

MFC after: 2 weeks