DaVinci does not support true I2S or right justified
mode so not all I2S codecs will work with it when the codec is
master. Document this limitation.

Add dsp_a, dsp_b mode options
Signed-off-by: NTroy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>