ASoC: dt-bindings: renesas,rsnd: simplify list of compatibles

The fallback compatible in a list (the last one) cannot be enum, because
it is always fixed.  Also if such fallback is used alone ("Generic"
case), it's not a list anymore.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230126100722.9473-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>