ASoC: add i2s dai driver for bf6xx soc

This driver enables i2s mode support on blackfin bf6xx platform.
We reuse bf5xx-i2s-pcm.c as its i2s pcm driver because it's the same
for both bf5xx and bf6xx soc.

Signed-off-by: Scott Jiang <scott.jiang.linux@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>