ASoC: dt-bindings: fsl_spdif: Document imx6sl/sx compatible fallback

ASoC: dt-bindings: fsl_spdif: Document imx6sl/sx compatible fallback

i.MX6SL and i.MX6SX SPDIF blocks are compatible with i.MX35.

Document 'fsl,imx35-spdif' as a fallback compatible for these two
chip variants.

This fixes the following dt-schema warnings:

compatible: ['fsl,imx6sl-spdif', 'fsl,imx35-spdif'] is too long
compatible: ['fsl,imx6sx-spdif', 'fsl,imx35-spdif'] is too long

Signed-off-by: Fabio Estevam <festevam@denx.de>
Link: https://patch.msgid.link/20241028020749.36972-1-festevam@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>