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

MCP3911 is a dual channel Analog Front End (AFE) containing two
synchronous sampling delta-sigma Analog-to-Digital Converters (ADC).
Co-Developed-by: Kent Gustavsson <kent@minoris.se>
Signed-off-by: Kent Gustavsson <kent@minoris.se>
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/adc/mcp3911.txt

+* 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>;
+};