ASoC: axi-i2s: make both "rx" and "tx" optional

Both the capture and playback channels are optional in the axi_i2s IP
block. Document how to tell the driver which channels are
instantiated.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Mark Brown <broonie@kernel.org>