dt-bindings: add mmio-based syscon mux controller DT bindings

MMIO register bitfield-based multiplexer controller bindings

Define register bitfields to be used to control multiplexers. The parent

device tree node must be a syscon node to provide register access.

Required properties:

- compatible : "mmio-mux"

- #mux-control-cells : <1>

- mux-reg-masks : an array of register offset and pre-shifted bitfield mask

                  pairs, each describing a single mux control.

* Standard mux-controller bindings as decribed in mux-controller.txt

Optional properties:

- idle-states : if present, the state the muxes will have when idle. The

		special state MUX_IDLE_AS_IS is the default.

The multiplexer state of each multiplexer is defined as the value of the

bitfield described by the corresponding register offset and bitfield mask pair

in the mux-reg-masks array, accessed through the parent syscon.

Example:

	syscon {

		compatible = "syscon";

		mux: mux-controller {

			compatible = "mmio-mux";

			#mux-control-cells = <1>;

			mux-reg-masks = <0x3 0x30>, /* 0: reg 0x3, bits 5:4 */

					<0x3 0x40>, /* 1: reg 0x3, bit 6 */

			idle-states = <MUX_IDLE_AS_IS>, <0>;

		};

	};

	video-mux {

		compatible = "video-mux";

		mux-controls = <&mux 0>;

		ports {

			/* inputs 0..3 */

			port@0 {

				reg = <0>;

			};

			port@1 {

				reg = <1>;

			};

			port@2 {

				reg = <2>;

			};

			port@3 {

				reg = <3>;

			};

			/* output */

			port@4 {

				reg = <4>;

			};

		};

	};