dt-bindings: iio: Add STM32 LPTimer trigger binding

Add documentation for STMicroelectronics STM32 Low-Power Timer Trigger
binding.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/iio/timer/stm32-lptimer-trigger.txt

+STMicroelectronics STM32 Low-Power Timer Trigger
+
+STM32 Low-Power Timer provides trigger source (LPTIM output) that can be used
+by STM32 internal ADC and/or DAC.
+
+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-trigger".
+- reg:			Identify trigger hardware block. Must be 0, 1 or 2
+			respectively for lptimer1, lptimer2 or lptimer3
+			trigger output.
+
+Example:
+	timer@40002400 {
+		compatible = "st,stm32-lptimer";
+		...
+		trigger@0 {
+			compatible = "st,stm32-lptimer-trigger";
+			reg = <0>;
+		};
+	};