Audio codec controlled by ChromeOS EC

Google's ChromeOS EC codec is a digital mic codec provided by the
Embedded Controller (EC) and is controlled via a host-command interface.

An EC codec node should only be found as a sub-node of the EC node (see
Documentation/devicetree/bindings/mfd/cros-ec.txt).

Required properties:
- compatible: Must contain "google,cros-ec-codec"
- #sound-dai-cells: Should be 1. The cell specifies number of DAIs.

Optional properties:
- reg: Pysical base address and length of shared memory region from EC.
       It contains 3 unsigned 32-bit integer.  The first 2 integers
       combine to become an unsigned 64-bit physical address.  The last
       one integer is length of the shared memory.
- memory-region: Shared memory region to EC.  A "shared-dma-pool".  See
                 ../reserved-memory/reserved-memory.txt for details.

Example:

{
	...

	reserved_mem: reserved_mem {
		compatible = "shared-dma-pool";
		reg = <0 0x52800000 0 0x100000>;
		no-map;
	};
}

cros-ec@0 {
	compatible = "google,cros-ec-spi";

	...

	cros_ec_codec: ec-codec {
		compatible = "google,cros-ec-codec";
		#sound-dai-cells = <1>;
		reg = <0x0 0x10500000 0x80000>;
		memory-region = <&reserved_mem>;
	};
};