ALSA: hda-intel - AD1984 thinkpad - add analog beep input control

For Lenovo Thinkpad T61/X61, the analog beep input is connected
to node 0x20, index 3. Move the digital beep mute/volume controls
as "Digital Beep" and create analog beep controls for mentioned node.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>