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

LTC1665/LTC1660 is a 8/10-bit Digital-to-Analog Converter (DAC)
with eight individual channels.
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/ltc1660.txt

+* 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>;
+};