Documentation: dt: iio: add st_lsm6dsx sensor device binding

Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/imu/st_lsm6dsx.txt

+* ST_LSM6DSx driver for STM 6-axis (acc + gyro) imu Mems sensors
+
+Required properties:
+- compatible: must be one of:
+  "st,lsm6ds3"
+  "st,lsm6dsm"
+- reg: i2c address of the sensor / spi cs line
+
+Optional properties:
+- interrupt-parent: should be the phandle for the interrupt controller
+- interrupts: interrupt mapping for IRQ. It should be configured with
+  flags IRQ_TYPE_LEVEL_HIGH or IRQ_TYPE_EDGE_RISING.
+
+  Refer to interrupt-controller/interrupts.txt for generic interrupt
+  client node bindings.
+
+Example:
+
+lsm6dsm@6b {
+	compatible = "st,lsm6dsm";
+	reg = <0x6b>;
+	interrupt-parent = <&gpio0>;
+	interrupts = <0 IRQ_TYPE_EDGE_RISING>;
+};