ASoC: dt-bindings: fsl-sai: allow only one dma-names

ASoC: dt-bindings: fsl-sai: allow only one dma-names

Some sai only connect one direction dma (rx/tx) in SOC. For example:
imx8qxp sai5 only connect tx dma channel. So allow only one "rx" or "tx"
for dma-names.

Remove description under dmas because no user use index to get dma channel.
All user use 'dma-names' to get correct dma channel. dma-names already in
'required' list.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://msgid.link/r/20240328-asrc_8qxp-v8-3-801cd6bb5be2@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>