dt-bindings: sound: sun4i-spdif: Document that the RX channel can be missing

The H3 and compatibles controllers don't have any reception capabilities,
even though it was never documented as such in the binding before.

Therefore, on those controllers, we don't have the option to set an RX DMA
channel.

This was already done in the DTSI, but the binding itself was never
updated. Let's add a special case in the schemas.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>