i2c: pca954x: Add device tree bindings documentation

* NXP PCA954x I2C bus switch

Required Properties:

  - compatible: Must contain one of the following.

    "nxp,pca9540", "nxp,pca9542", "nxp,pca9543", "nxp,pca9544",

    "nxp,pca9545", "nxp,pca9546", "nxp,pca9547", "nxp,pca9548"

  - reg: The I2C address of the device.

  The following required properties are defined externally:

  - Standard I2C mux properties. See i2c-mux.txt in this directory.

  - I2C child bus nodes. See i2c-mux.txt in this directory.

Example:

	i2c-switch@74 {

		compatible = "nxp,pca9548";

		#address-cells = <1>;

		#size-cells = <0>;

		reg = <0x74>;

		i2c@2 {

			#address-cells = <1>;

			#size-cells = <0>;

			reg = <2>;

			eeprom@54 {

				compatible = "at,24c08";

				reg = <0x54>;

			};

		};

		i2c@4 {

			#address-cells = <1>;

			#size-cells = <0>;

			reg = <4>;

			rtc@51 {

				compatible = "nxp,pcf8563";

				reg = <0x51>;

			};

		};

	};