Documentation: dt-bindings: Document bindings for Aspeed ADC

Signed-off-by: Rick Altherr <raltherr@google.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/adc/aspeed_adc.txt

+Aspeed ADC
+
+This device is a 10-bit converter for 16 voltage channels.  All inputs are
+single ended.
+
+Required properties:
+- compatible: Should be "aspeed,ast2400-adc" or "aspeed,ast2500-adc"
+- reg: memory window mapping address and length
+- clocks: Input clock used to derive the sample clock. Expected to be the
+          SoC's APB clock.
+- #io-channel-cells: Must be set to <1> to indicate channels are selected
+                     by index.
+
+Example:
+	adc@1e6e9000 {
+		compatible = "aspeed,ast2400-adc";
+		reg = <0x1e6e9000 0xb0>;
+		clocks = <&clk_apb>;
+		#io-channel-cells = <1>;
+	};