GD ROM DMA support

[Arno500]

Hello there!

I was curious about what it would take to properly support DMA syscalls to improve GD ROM drive emulation.
Any thoughts?

PS: I would be willing to take some time to implement it, but:

1. I am pretty bad at C
2. I did not do much low level/embedded programming (but some!)
3. I never touched programming on/about the Dreamcast (but I do own a real PAL one)