[ARM] pxa: move DMA registers definitions into <mach/dma.h>

1. Driver code where pxa_request_dma() is called will most likely
   reference DMA registers as well,  and it is really unnecessary
   to include pxa-regs.h just for this. Move the definitions into
   <mach/dma.h> and make relevant drivers include it instead of
   <mach/pxa-regs.h>.

2. Introduce DMAC_REGS_VIRT as the virtual address base for these
   DMA registers. This allows later processors to re-use the same
   IP while registers may start at different I/O address.

Signed-off-by: Eric Miao <eric.miao@marvell.com>