[ALSA] Stereo controls for M-Audio Revolution cards

This patch adds stereo controls to revo cards by making the ak4xxx
driver mixers configurable from the card driver.

Signed-off-by: Jani Alinikula <janialinikula@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>