dt-bindings: iio: stm32-lptimer-counter: document pinctrl sleep state

Add documentation for optional pinctrl sleep state that can be used by
STM32 LPTimer encoder/counter.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -10,8 +10,9 @@ 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,
+- pinctrl-names: 	Set to "default". An additional "sleep" state can be
+			defined to set pins in sleep state.
+- pinctrl-n: 		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.
 
@@ -21,7 +22,8 @@ Example:
 		...
 		counter {
 			compatible = "st,stm32-lptimer-counter";
-			pinctrl-names = "default";
+			pinctrl-names = "default", "sleep";
 			pinctrl-0 = <&lptim1_in_pins>;
+			pinctrl-1 = <&lptim1_sleep_in_pins>;
 		};
 	};