media: atmel: atmel-isc: specialize dma cfg

The dma configuration (DCFG) is specific to the product.
Move this configuration in the product specific driver, and add the
field inside the driver struct.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>