ASoC: dpcm: add rate merge to the BE stream merge

As done for format and channels, add the possibility to merge
the backend rates on the frontend rates.

This useful if the backend does not support all rates supported by the
frontend, or if several backends (cpu and codecs) with different
capabilities are connected to the same frontend.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>