Rotary encoder DT bindings

Required properties:
- gpios: a spec for two GPIOs to be used

Optional properties:
- linux,axis: the input subsystem axis to map to this rotary encoder.
  Defaults to 0 (ABS_X / REL_X)
- rotary-encoder,steps: Number of steps in a full turnaround of the
  encoder. Only relevant for absolute axis. Defaults to 24 which is a
  typical value for such devices.
- rotary-encoder,relative-axis: register a relative axis rather than an
  absolute one. Relative axis will only generate +1/-1 events on the input
  device, hence no steps need to be passed.
- rotary-encoder,rollover: Automatic rollove when the rotary value becomes
  greater than the specified steps or smaller than 0. For absolute axis only.
- rotary-encoder,half-period: Makes the driver work on half-period mode.

See Documentation/input/rotary-encoder.txt for more information.

Example:

		rotary@0 {
			compatible = "rotary-encoder";
			gpios = <&gpio 19 1>, <&gpio 20 0>; /* GPIO19 is inverted */
			linux,axis = <0>; /* REL_X */
			rotary-encoder,relative-axis;
		};

		rotary@1 {
			compatible = "rotary-encoder";
			gpios = <&gpio 21 0>, <&gpio 22 0>;
			linux,axis = <1>; /* ABS_Y */
			rotary-encoder,steps = <24>;
			rotary-encoder,rollover;
		};
