dmaengine: omap-dma: move register read/writes into omap-dma.c

Export the DMA register information from the SoC specific data, such
that we can access the registers directly in omap-dma.c, mapping the
register region ourselves as well.

Rather than calculating the DMA channel register in its entirety for
each access, we pre-calculate an offset base address for the allocated
DMA channel and then just use the appropriate register offset.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>