pcm: rate: Add capability to pass configuration node to plugins

pcm: rate: Add capability to pass configuration node to plugins

If a rate plugin uses a node (compound) instead of a plain string for
its "converter", and that compound is not a simple string array, then
the compound will be passed as an additional parameter to the new plugin
open() function (SND_PCM_RATE_PLUGIN_CONF_ENTRY(XXX)). The previous
open() function (SND_PCM_RATE_PLUGIN_ENTRY(XXX)) will be called if the
CONF version is not found. It is up to the plugin to determine whether
the presence of the conf parameter is mandatory.

Signed-off-by: Alan Young <consult.awy@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>