ASoC: AD1980 codec: add multi-channel function support

We added multi-channel function to this codec driver and Blackfin ASoC driver as well.
It was tested on Blackfin hardware.

Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>