[ARM] S3C24XX: DMA: Split hardware regs out of <mach/dma.h>

The <mach/dma.h> is for the driver API for the DMA system and should
not have anything relying on the CPU specific registers.

Remove the registers to <plat/dma-regs.h> for the code that really
needs to know about them.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>