Input: silead - add pen support

Some Silead touchscreens have support for an active (battery powered)
pen, add support for this.

So far pen-support has only been seen on X86/ACPI (non devicetree) devs,
IOW it is not used in actual devicetree files. The devicetree-bindings
maintainers have requested properties like these to not be added to the
devicetree-bindings, so the new properties are deliberately not added
to the existing silead devicetree-bindings documentation.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20211122220637.11386-3-hdegoede@redhat.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>