ideapad_laptop: Add an event for mic mute hotkey

Newer ideapads support a new mic hotkey implemented via an ACPI
interface. This patch converts the mic mute event to a keycode
KEY_MICMUTE.

Signed-off-by: Alex Hung <alex.hung@canonical.com>
Acked-by: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>