thinkpad-acpi: basic ALSA mixer support (v2)

Add the basic ALSA mixer functionality.  The mixer is event-driven,
and will work fine on IBM ThinkPads.  I expect Lenovo ThinkPads will
cause some trouble with the event interface.

Heavily based on work by Lorne Applebaum <lorne.applebaum@gmail.com>
and ideas from Matthew Garrett <mjg@redhat.com>.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Lorne Applebaum <lorne.applebaum@gmail.com>
Cc: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>