sound: oxygen: add stereo upmixing to center/LFE channels

Add the possibility to route a mix of the two channels of stereo data to
the center and LFE outputs.  This is implemented only for models where
the DACs support this, i.e., for the Xonar D1 and DX.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>