Add generic driver for Silead tochscreens

Add generic driver for Silead tochscreens

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

It supports ACPI and device tree enumeration. For ACPI you need ACPI 5.1+
in order to be able to use named GPIOs. In device tree you can use 'interrupts'
or a named GPIO for IRQ.

Screen resolution, the maximum number of fingers supported and firmware name
are configurable using ACPI/DT properties.

Signed-off-by: Robert Dolca <robert.dolca@intel.com>