ASoC: tse850: document axentia,tse850-pcm5142 bindings

The TSE-850 is an FM Transmitter Station Equipment, designed to generate
baseband signals for FM, mainly the DARC subcarrier, but other signals
are also possible.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Mark Brown <broonie@kernel.org>