dt-bindings: iio: Add STM32 LPTimer quadrature encoder and counter

STMicroelectronics STM32 Low-Power Timer quadrature encoder and counter

STM32 Low-Power Timer provides several counter modes. It can be used as:

- quadrature encoder to detect angular position and direction of rotary

  elements, from IN1 and IN2 input signals.

- simple counter from IN1 input signal.

Must be a sub-node of an STM32 Low-Power Timer device tree node.

See ../mfd/stm32-lptimer.txt for details about the parent node.

Required properties:

- compatible:		Must be "st,stm32-lptimer-counter".

- pinctrl-names: 	Set to "default".

- pinctrl-0: 		List of phandles pointing to pin configuration nodes,

			to set IN1/IN2 pins in mode of operation for Low-Power

			Timer input on external pin.

Example:

	timer@40002400 {

		compatible = "st,stm32-lptimer";

		...

		counter {

			compatible = "st,stm32-lptimer-counter";

			pinctrl-names = "default";

			pinctrl-0 = <&lptim1_in_pins>;

		};

	};