I'm now looking seriously (and reluctantly!) at writing a DMA Controller
Driver to supplement these functions, and I've started the process of
getting an NDA from Marvell, so I can get their User Manual (and errata!).

You won't get very far without the user manual, then I think you will find it 
pretty easy to write the driver with the frameworks already in the kernel.

Can someone point me in the right direction to get me started?  I need
to come up the learning curve to find out where to start.

How is a DMA controlled (from a device driver writer's perspective) when
a third-party (i.e. in the bridge) DMA controller needs to do the work
to get the data from a PCI Target into main memory?

What kernel API should be provided by the DMA Controller Driver?

My first guess would be to follow something like Documentation/DMA-API.txt and 
Documentation/DMA-mapping.txt

Any documentation, examples, similar device drivers, etc, would be
appreciated.  TIA,

You could look to followup that by groking arch/ppc/kernel/dma-mapping.c
Finally, arch/ppc/syslib ppc4xx_dma.c seems to show an example of a low level 
driver. 
I didn't notice any platform dma controllers like MAG reccommended, but that 
should be a better way to go. 

Thanks
Brian