Input: add driver for Silead touchscreens

This driver adds support for Silead touchscreens. It has been tested
with GSL1680 and GSL3680 touch panels.

It supports ACPI and device tree enumeration. Screen resolution,
the maximum number of fingers supported and firmware name are
configurable.

Signed-off-by: Robert Dolca <robert.dolca@intel.com>
Signed-off-by: Daniel Jansen <djaniboe@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>