Device-Tree bindings for GPIO attached mice

This simply uses standard GPIO handles to define a simple mouse connected
to 5-7 GPIO lines.

Required properties:
	- compatible: must be "gpio-mouse"
	- scan-interval-ms: The scanning interval in milliseconds
	- up-gpios: GPIO line phandle to the line indicating "up"
	- down-gpios: GPIO line phandle to the line indicating "down"
	- left-gpios: GPIO line phandle to the line indicating "left"
	- right-gpios: GPIO line phandle to the line indicating "right"

Optional properties:
	- button-left-gpios: GPIO line handle to the left mouse button
	- button-middle-gpios: GPIO line handle to the middle mouse button
	- button-right-gpios: GPIO line handle to the right mouse button
Example:

#include <dt-bindings/gpio/gpio.h>

gpio-mouse {
	compatible = "gpio-mouse";
	scan-interval-ms = <50>;
	up-gpios = <&gpio0 0 GPIO_ACTIVE_LOW>;
	down-gpios = <&gpio0 1 GPIO_ACTIVE_LOW>;
	left-gpios = <&gpio0 2 GPIO_ACTIVE_LOW>;
	right-gpios = <&gpio0 3 GPIO_ACTIVE_LOW>;
	button-left-gpios = <&gpio0 4 GPIO_ACTIVE_LOW>;
	button-middle-gpios = <&gpio0 5 GPIO_ACTIVE_LOW>;
	button-right-gpios = <&gpio0 6 GPIO_ACTIVE_LOW>;
};