HID: wacom: Add support for Express Key Remote.

This device is pad (buttons) only, there is no stylus or touch. Up to
five remotes can pair with the device's associated USB dongle.

Signed-off-by: Aaron Skomra <aaron.skomra@wacom.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>