dmaengine: bcm2835: add additional defines for DMA-registers

Add additional defines describing the DMA registers
as well as adding some more documentation to those registers.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>