Input: hgpk - support GlideSensor and PenTablet modes

Add a "hgpk_mode" sysfs attribute that allows selection between 3 options:
Mouse (the existing option), GlideSensor and PenTablet.

GlideSensor is an enhanced protocol for the regular touchpad mode that
additionally reports pressure and uses absolute coordinates. We suspect
that it may be more reliable than mouse mode in some environments.

PenTablet mode puts the touchpad into resistive mode, you must then use
a stylus as an input. We suspect this is the most reliable way to drive
the touchpad.

The GlideSensor and PenTablet devices expose themselves with the
intention of being combined with the synaptics X11 input driver.

Based on earlier work by Paul Fox.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>