ppc440_uc: Basic emulation of PPC440 DMA controller

PPC440 SoCs such as the AMCC 460EX have a DMA controller which is used
by AmigaOS on the sam460ex. Implement the parts used by AmigaOS so it
can get further booting on the sam460ex machine.

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