dt-bindings: counter: Document stm32 quadrature encoder

Add bindings for STM32 Timer quadrature encoder.
It is a sub-node of STM32 Timer which implement the
quadratic encoder part of the hardware.

Cc: Mark Rutland <[e-mail]>
Signed-off-by: Benjamin Gaignard <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: William Breathitt Gray <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/counter/stm32-timer-cnt.txt

···
+STMicroelectronics STM32 Timer quadrature encoder
+
+STM32 Timer provides quadrature encoder to detect
+angular position and direction of rotary elements,
+from IN1 and IN2 input signals.
+
+Must be a sub-node of an STM32 Timer device tree node.
+See ../mfd/stm32-timers.txt for details about the parent node.
+
+Required properties:
+- compatible:		Must be "st,stm32-timer-counter".
+- pinctrl-names: 	Set to "default".
+- pinctrl-0: 		List of phandles pointing to pin configuration nodes,
+			to set CH1/CH2 pins in mode of operation for STM32
+			Timer input on external pin.
+
+Example:
+	timers@40010000 {
+		#address-cells = <1>;
+		#size-cells = <0>;
+		compatible = "st,stm32-timers";
+		reg = <0x40010000 0x400>;
+		clocks = <&rcc 0 160>;
+		clock-names = "int";
+
+		counter {
+			compatible = "st,stm32-timer-counter";
+			pinctrl-names = "default";
+			pinctrl-0 = <&tim1_in_pins>;
+		};
+	};

Documentation/devicetree/bindings/mfd/stm32-timers.txt

···
 Optional subnodes:
 - pwm:			See ../pwm/pwm-stm32.txt
 - timer:		See ../iio/timer/stm32-timer-trigger.txt
+- counter:		See ../counter/stm32-timer-cnt.txt
 
 Example:
 	timers@40010000 {
···
 		timer@0 {
 			compatible = "st,stm32-timer-trigger";
 			reg = <0>;
+		};
+
+		counter {
+			compatible = "st,stm32-timer-counter";
+			pinctrl-names = "default";
+			pinctrl-0 = <&tim1_in_pins>;
 		};
 	};