ALSA: compress: add raw opus codec define and opus decoder structs

ALSA: compress: add raw opus codec define and opus decoder structs

Adds a raw opus codec define and raw opus decoder structs.
This is for raw OPUS packets not packed in any type of container
(for instance OGG container). The decoder struct fields are
taken from corresponding RFC document: RFC 7845 Section 5.

Cc: Srinivas Kandagatla <srini@kernel.org>
Cc: Vinod Koul <vkoul@kernel.org>
Co-developed-by: Annemarie Porter <annemari@quicinc.com>
Signed-off-by: Annemarie Porter <annemari@quicinc.com>
Signed-off-by: Alexey Klimov <alexey.klimov@linaro.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>