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>;
	};