dt-bindings: iio: adc: add bindings for mcp3911

* Microchip MCP3911 Dual channel analog front end (ADC)

Required properties:

 - compatible: Should be "microchip,mcp3911"

 - reg: SPI chip select number for the device

Recommended properties:

 - spi-max-frequency: Definition as per

	 Documentation/devicetree/bindings/spi/spi-bus.txt.

	 Max frequency for this chip is 20MHz.

Optional properties:

 - clocks: Phandle and clock identifier for sampling clock

 - interrupt-parent: Phandle to the parent interrupt controller

 - interrupts: IRQ line for the ADC

 - microchip,device-addr: Device address when multiple MCP3911 chips are present on the

	same SPI bus. Valid values are 0-3. Defaults to 0.

 - vref-supply: Phandle to the external reference voltage supply.

Example:

adc@0 {

	compatible = "microchip,mcp3911";

	reg = <0>;

	interrupt-parent = <&gpio5>;

	interrupts = <15 IRQ_TYPE_EDGE_RISING>;

	spi-max-frequency = <20000000>;

	microchip,device-addr = <0>;

	vref-supply = <&vref_reg>;

	clocks = <&xtal>;

};