PPC: g3beige:  Move secondary IDE bus to mac-io

On a real G3 Beige the secondary IDE bus lives on the mac-io chip, not
on some random PCI device. Move it there to become more compatible.

While at it, also clean up the IDE channel connection logic.

Signed-off-by: Alexander Graf <agraf@suse.de>

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v1 -> v2:

  - fix IRQ mapping
XXX openbios
XXX dbdma debug