dt: document NXP LPC1850 DAC driver bindings

NXP LPC1850 DAC bindings

Required properties:

- compatible: Should be "nxp,lpc1850-dac"

- reg: Offset and length of the register set for the ADC device

- interrupts: The interrupt number for the ADC device

- clocks: The root clock of the ADC controller

- vref-supply: The regulator supply ADC reference voltage

- resets: phandle to reset controller and line specifier

Example:

dac: dac@400e1000 {

	compatible = "nxp,lpc1850-dac";

	reg = <0x400e1000 0x1000>;

	interrupts = <0>;

	clocks = <&ccu1 CLK_APB3_DAC>;

	vref-supply = <&reg_vdda>;

	resets = <&rgu 42>;

	status = "disabled";

};