ASoC: dt-bindings: marvell, mmp-sspa: Use audio-graph-port schema

Now that we have a graph schema, reference it from the marvell,mmp-sspa
schema.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20201117013349.2458416-4-robh@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>