iio: ad5755: Add DT binding documentation

Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/dac/ad5755.txt

+* Analog Device AD5755 IIO Multi-Channel DAC Linux Driver
+
+Required properties:
+ - compatible: Has to contain one of the following:
+	adi,ad5755
+	adi,ad5755-1
+	adi,ad5757
+	adi,ad5735
+	adi,ad5737
+
+ - reg: spi chip select number for the device
+ - spi-cpha or spi-cpol: is the only modes that is supported
+
+Recommended properties:
+ - spi-max-frequency: Definition as per
+		Documentation/devicetree/bindings/spi/spi-bus.txt
+
+Optional properties:
+See include/dt-bindings/iio/ad5755.h
+ - adi,ext-dc-dc-compenstation-resistor: boolean set if the hardware have an
+					 external resistor and thereby bypasses
+					 the internal compensation resistor.
+ - adi,dc-dc-phase:
+	Valid values for DC DC Phase control is:
+	0: All dc-to-dc converters clock on the same edge.
+	1: Channel A and Channel B clock on the same edge,
+	   Channel C and Channel D clock on opposite edges.
+	2: Channel A and Channel C clock on the same edge,
+	   Channel B and Channel D clock on opposite edges.
+	3: Channel A, Channel B, Channel C, and Channel D
+	   clock 90 degrees out of phase from each other.
+ - adi,dc-dc-freq-hz:
+	Valid values for DC DC frequency is [Hz]:
+	250000
+	410000
+	650000
+ - adi,dc-dc-max-microvolt:
+	Valid values for the maximum allowed Vboost voltage supplied by
+	the dc-to-dc converter is:
+	23000000
+	24500000
+	27000000
+	29500000
+
+Optional for every channel:
+ - adi,mode:
+	Valid values for DAC modes is:
+	0: 0 V to 5 V voltage range.
+	1: 0 V to 10 V voltage range.
+	2: Plus minus 5 V voltage range.
+	3: Plus minus 10 V voltage range.
+	4: 4 mA to 20 mA current range.
+	5: 0 mA to 20 mA current range.
+	6: 0 mA to 24 mA current range.
+ - adi,ext-current-sense-resistor: boolean set if the hardware a external
+				   current sense resistor.
+ - adi,enable-voltage-overrange: boolean enable voltage overrange
+ - adi,slew: Array of slewrate settings should contain 3 fields:
+	1: Should be either 0 or 1 in order to enable or disable slewrate.
+	2: Slew rate settings:
+		Valid values for the slew rate update frequency:
+		64000
+		32000
+		16000
+		8000
+		4000
+		2000
+		1000
+		500
+		250
+		125
+		64
+		32
+		16
+		8
+		4
+		0
+	3: Slew step size:
+		Valid values for the step size LSBs:
+		1
+		2
+		4
+		16
+		32
+		64
+		128
+		256
+
+Example:
+dac@0 {
+	#address-cells = <1>;
+	#size-cells = <0>;
+	compatible = "adi,ad5755";
+	reg = <0>;
+	spi-max-frequency = <1000000>;
+	spi-cpha;
+	adi,dc-dc-phase = <0>;
+	adi,dc-dc-freq-hz = <410000>;
+	adi,dc-dc-max-microvolt = <23000000>;
+	channel@0 {
+		reg = <0>;
+		adi,mode = <4>;
+		adi,ext-current-sense-resistor;
+		adi,slew = <0 64000 1>;
+	};
+	channel@1 {
+		reg = <1>;
+		adi,mode = <4>;
+		adi,ext-current-sense-resistor;
+		adi,slew = <0 64000 1>;
+	};
+	channel@2 {
+		reg = <2>;
+		adi,mode = <4>;
+		adi,ext-current-sense-resistor;
+		adi,slew = <0 64000 1>;
+	};
+	channel@3 {
+		reg = <3>;
+		adi,mode = <4>;
+		adi,ext-current-sense-resistor;
+		adi,slew = <0 64000 1>;
+	};
+};