dmaengine: omap-dma: provide register read/write functions

Provide a pair of channel register accessors, and a pair of global
accessors for non-channel specific registers.

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>