Input: wacom - advertise BTN_TOOL_PEN and BTN_STYLUS for PenPartner

The Wacom PenPartner should advertise its stylus tip and button
in addition to the eraser tool. These are both physically
present on the hardware, and emitted from 'wacom_penpartner_irq'.

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