Groundwork for IOP DMA implementation

* The DMA routine on the IOP works similarly to the PSX version with a
few additions. There are 7 channels from the PSX and an additional 6 new
PS2 exclusive ones. One the PSX, each channel has 3 registers used
to configure and use it and 3 global registers.

* The PS2 contains all the older DMA registers, but it add 6 more channels
and duplicates the global registers (DPCR now has a counterpart called DPCR2)
This is done because each global register can control up to 7 channels.
An additional register on each channel (tadr) and 2 additional
global registers have been added as well. For now we don't really
care to implement them, only read/write to them.

* For reading and writing to the registers structs are used to prevent
the usage of switch and if statements.