[Blackfin] arch: Give the DMA base registers a more descriptive name

The DMA base registers are available in a global named "base_addr" for
every Blackfin variant. Give this a more descriptive name, and remove
duplicate tables from some drivers.

Signed-off-by: Bernd Schmidt <bernds_cb1@t-online.de>
Signed-off-by: Bryan Wu <cooloney@kernel.org>