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

Add documentation for STMicroelectronics STM32 Low-Power Timer
quadrature encoder and counter binding.
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: NJonathan Cameron <jic23@kernel.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/iio/counter/stm32-lptimer-cnt.txt

+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>;
+		};
+	};