* GSL 1680 touchscreen controller

Required properties:
- 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)
- interrupts		  : interrupt specification for the gsl1680 interrupt
- power-gpios		  : Specification for the pin connected to the gsl1680's
			    shutdown input. This needs to be driven high to take the
			    gsl1680 out of its low power state
- touchscreen-size-x	  : See touchscreen.txt
- touchscreen-size-y	  : See touchscreen.txt

Optional properties:
- firmware-name		  : File basename (string) for board specific firmware
- touchscreen-inverted-x  : See touchscreen.txt
- touchscreen-inverted-y  : See touchscreen.txt
- touchscreen-swapped-x-y : See touchscreen.txt
- silead,max-fingers	  : maximum number of fingers the touchscreen can detect
- silead,home-button	  : Boolean, set to true on devices which have a
			    capacitive home-button build into the touchscreen
- vddio-supply		  : regulator phandle for controller VDDIO
- avdd-supply		  : regulator phandle for controller AVDD

Example:

i2c@00000000 {
	gsl1680: touchscreen@40 {
		compatible = "silead,gsl1680";
		reg = <0x40>;
		interrupt-parent = <&pio>;
		interrupts = <6 11 IRQ_TYPE_EDGE_FALLING>;
		power-gpios = <&pio 1 3 GPIO_ACTIVE_HIGH>;
		touchscreen-size-x = <480>;
		touchscreen-size-y = <800>;
		touchscreen-inverted-x;
		touchscreen-swapped-x-y;
		silead,max-fingers = <5>;
	};
};