Add a DT schema for the SSM3515 amp by Analog Devices. It's a simple
mono amplifier with digital input.

Signed-off-by: Martin Povišer <povik+lin@cutebit.org
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org
Link: https://lore.kernel.org/r/20230511150546.8499-2-povik+lin@cutebit.org
Signed-off-by: Mark Brown <broonie@kernel.org