dt-binding: iio: add NPCM ADC documentation

Added device tree binding documentation for Nuvoton BMC
NPCM Analog-to-Digital Converter(ADC).
Signed-off-by: NTomer Maimon <tmaimon77@gmail.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/nuvoton,npcm-adc.txt

+Nuvoton NPCM Analog to Digital Converter (ADC)
+
+The NPCM ADC is a 10-bit converter for eight channel inputs.
+
+Required properties:
+- compatible: "nuvoton,npcm750-adc" for the NPCM7XX BMC.
+- reg: specifies physical base address and size of the registers.
+- interrupts: Contain the ADC interrupt with flags for falling edge.
+
+Optional properties:
+- clocks: phandle of ADC reference clock, in case the clock is not
+		  added the ADC will use the default ADC sample rate.
+- vref-supply: The regulator supply ADC reference voltage, in case the
+			   vref-supply is not added the ADC will use internal voltage
+			   reference.
+
+Required Node in the NPCM7xx BMC:
+An additional register is present in the NPCM7xx SOC which is
+assumed to be in the same device tree, with and marked as
+compatible with "nuvoton,npcm750-rst".
+
+Example:
+
+adc: adc@f000c000 {
+	compatible = "nuvoton,npcm750-adc";
+	reg = <0xf000c000 0x8>;
+	interrupts = <GIC_SPI 0 IRQ_TYPE_LEVEL_HIGH>;
+	clocks = <&clk NPCM7XX_CLK_ADC>;
+};
+
+rst: rst@f0801000 {
+	compatible = "nuvoton,npcm750-rst", "syscon",
+	"simple-mfd";
+	reg = <0xf0801000 0x6C>;
+};