dt-bindings: iio: dac: add bindings for ltc1660

* Linear Technology Micropower octal 8-Bit and 10-Bit DACs

Required properties:

 - compatible: Must be one of the following:

		"lltc,ltc1660"

		"lltc,ltc1665"

 - reg: SPI chip select number for the device

 - vref-supply: Phandle to the voltage reference supply

Recommended properties:

 - spi-max-frequency: Definition as per

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

	 Max frequency for this chip is 5 MHz.

Example:

dac@0 {

	compatible = "lltc,ltc1660";

	reg = <0>;

	spi-max-frequency = <5000000>;

	vref-supply = <&vref_reg>;

};