ASoC: rsnd: Document SoC-specific bindings

The documentation only mentioned the generic fallback compatible
property.
Add the missing SoC-specific compatible properties, which are already in
use.

Also drop a bogus 0x unit-address prefix while we're at it.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mark Brown <broonie@kernel.org>