Input: wacom - add support for Cintiq 24HD

Adds support for the Cintiq 24HD. There are two quirks about this
model that haven't been seen in prior tablets. First, a second
touch ring is present on this display; it is being exposed via the
ABS_THROTTLE axis. Second, three capacitive buttons at the top of
the unit are available; though physically a touch strip, we report
the use of these buttons with generic KEY_ events.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>