dt-bindings: sound: sun4i-i2s: Document that the RX channel can be missing

The A83t and compatibles controllers don't have any reception capabilities
on some instances of the controllers, 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>