dt-bindings: asoc: adau7118: Cleanup

This changes are in accordance with the review done to this bindings.
This is a follow-up patch to 969d49b2

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Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20191021140816.262401-2-nuno.sa@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>