ASoC: dt-bindings: fsl-sai: Support Rx-only SAI

On SoC like i.MX8QXP some SAI IP cores support only Rx path. Allow
specifying only an Rx DMA channel, while omitting the Tx one.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://msgid.link/r/20231219122957.3358149-2-alexander.stein@ew.tq-group.com


Signed-off-by: Mark Brown <broonie@kernel.org>