Analog Devices AD5758 DAC device driver

Required properties for the AD5758:
	- compatible: Must be "adi,ad5758"
	- reg: SPI chip select number for the device
	- spi-max-frequency: Max SPI frequency to use (< 50000000)
	- spi-cpha: is the only mode that is supported

Required properties:

 - adi,dc-dc-mode: Mode of operation of the dc-to-dc converter
		   Dynamic Power Control (DPC)
		   In this mode, the AD5758 circuitry senses the output
		   voltage and dynamically regulates the supply voltage,
		   VDPC+, to meet compliance requirements plus an optimized
		   headroom voltage for the output buffer.

		   Programmable Power Control (PPC)
		   In this mode, the VDPC+ voltage is user-programmable to
		   a fixed level that needs to accommodate the maximum output
		   load required.

		   The output of the DAC core is either converted to a
		   current or voltage output at the VIOUT pin. Only one mode
		   can be enabled at any one time.

		   The following values are currently supported:
			* 1: DPC current mode
			* 2: DPC voltage mode
			* 3: PPC current mode

 Depending on the selected output mode (voltage or current) one of the
 two properties must
 be present:

 - adi,range-microvolt: Voltage output range
		The array of voltage output ranges must contain two fields:
		* <0 5000000>: 0 V to 5 V voltage range
		* <0 10000000>: 0 V to 10 V voltage range
		* <(-5000000) 5000000>: ±5 V voltage range
		* <(-10000000) 10000000>: ±10 V voltage range
 - adi,range-microamp: Current output range
		The array of current output ranges must contain two fields:
		* <0 20000>: 0 mA to 20 mA current range
		* <0 24000>: 0 mA to 24 mA current range
		* <4 24000>: 4 mA to 20 mA current range
		* <(-20000) 20000>: ±20 mA current range
		* <(-24000) 24000>: ±24 mA current range
		* <(-1000) 22000>: −1 mA to +22 mA current range

Optional properties:

 - reset-gpios : GPIO spec for the RESET pin. If specified, it will be
		 asserted during driver probe.

 - adi,dc-dc-ilim-microamp: The dc-to-dc converter current limit
		   The following values are currently supported [uA]:
			* 150000
			* 200000
			* 250000
			* 300000
			* 350000
			* 400000

 - adi,slew-time-us: The time it takes for the output to reach the
 full scale [uS]
		     The supported range is between 133us up to 1023984375us

AD5758 Example:

	dac@0 {
		compatible = "adi,ad5758";
		reg = <0>;
		spi-max-frequency = <1000000>;
		spi-cpha;

		reset-gpios = <&gpio 22 0>;

		adi,dc-dc-mode = <2>;
		adi,range-microvolt = <0 10000000>;
		adi,dc-dc-ilim-microamp = <200000>;
		adi,slew-time-us = <125000>;
	};