Input: silead - list all supported compatible strings in binding document

The driver contains compatible strings for different models, but the DT
binding doc only lists one of them. Add the remaining to the document.
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/touchscreen/silead_gsl1680.txt

 * GSL 1680 touchscreen controller
 
 Required properties:
-- compatible		  : "silead,gsl1680"
+- compatible		  : Must be one of the following, depending on the model:
+			    "silead,gsl1680"
+			    "silead,gsl1688"
+			    "silead,gsl3670"
+			    "silead,gsl3675"
+			    "silead,gsl3692"
 - reg			  : I2C slave address of the chip (0x40)
 - interrupt-parent	  : a phandle pointing to the interrupt controller
 			    serving the interrupt for this chip