Pinctrl-based I2C Bus Mux

This binding describes an I2C bus multiplexer that uses pin multiplexing to
route the I2C signals, and represents the pin multiplexing configuration
using the pinctrl device tree bindings.

                                 +-----+  +-----+
                                 | dev |  | dev |
    +------------------------+   +-----+  +-----+
    | SoC                    |      |        |
    |                   /----|------+--------+
    |   +---+   +------+     | child bus A, on first set of pins
    |   |I2C|---|Pinmux|     |
    |   +---+   +------+     | child bus B, on second set of pins
    |                   \----|------+--------+--------+
    |                        |      |        |        |
    +------------------------+  +-----+  +-----+  +-----+
                                | dev |  | dev |  | dev |
                                +-----+  +-----+  +-----+

Required properties:
- compatible: i2c-mux-pinctrl
- i2c-parent: The phandle of the I2C bus that this multiplexer's master-side
  port is connected to.

Also required are:

* Standard pinctrl properties that specify the pin mux state for each child
  bus. See ../pinctrl/pinctrl-bindings.txt.

* Standard I2C mux properties. See i2c-mux.yaml in this directory.

* I2C child bus nodes. See i2c-mux.yaml in this directory.

For each named state defined in the pinctrl-names property, an I2C child bus
will be created. I2C child bus numbers are assigned based on the index into
the pinctrl-names property.

The only exception is that no bus will be created for a state named "idle". If
such a state is defined, it must be the last entry in pinctrl-names. For
example:

	pinctrl-names = "ddc", "pta", "idle"  ->  ddc = bus 0, pta = bus 1
	pinctrl-names = "ddc", "idle", "pta"  ->  Invalid ("idle" not last)
	pinctrl-names = "idle", "ddc", "pta"  ->  Invalid ("idle" not last)

Whenever an access is made to a device on a child bus, the relevant pinctrl
state will be programmed into hardware.

If an idle state is defined, whenever an access is not being made to a device
on a child bus, the idle pinctrl state will be programmed into hardware.

If an idle state is not defined, the most recently used pinctrl state will be
left programmed into hardware whenever no access is being made of a device on
a child bus.

Example:

	i2cmux {
		compatible = "i2c-mux-pinctrl";
		#address-cells = <1>;
		#size-cells = <0>;

		i2c-parent = <&i2c1>;

		pinctrl-names = "ddc", "pta", "idle";
		pinctrl-0 = <&state_i2cmux_ddc>;
		pinctrl-1 = <&state_i2cmux_pta>;
		pinctrl-2 = <&state_i2cmux_idle>;

		i2c@0 {
			reg = <0>;
			#address-cells = <1>;
			#size-cells = <0>;

			eeprom {
				compatible = "eeprom";
				reg = <0x50>;
			};
		};

		i2c@1 {
			reg = <1>;
			#address-cells = <1>;
			#size-cells = <0>;

			eeprom {
				compatible = "eeprom";
				reg = <0x50>;
			};
		};
	};