Documentation: dt: iio: add st_lsm6dsx sensor device binding

* 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>;

};