dt-bindings: interconnect: Document the support of optional path tag

Required properties:

- compatible : contains the interconnect provider compatible string

- #interconnect-cells : number of cells in a interconnect specifier needed to

			encode the interconnect node id

			encode the interconnect node id and optionally add a

			path tag

Example:

Required properties:

interconnects : Pairs of phandles and interconnect provider specifier to denote

	        the edge source and destination ports of the interconnect path.

		An optional path tag value could specified as additional argument

		to both endpoints and in such cases, this information will be passed

		to the interconnect framework to do aggregation based on the attached

		tag.

Optional properties:

interconnect-names : List of interconnect path name strings sorted in the same

		interconnects = <&pnoc MASTER_SDCC_1 &bimc SLAVE_EBI_CH0>;

		interconnect-names = "sdhc-mem";

	};

Example with path tags:

	gnoc: interconnect@17900000 {

		...

		interconnect-cells = <2>;

	};

	mnoc: interconnect@1380000 {

		...

		interconnect-cells = <2>;

	};

	cpu@0 {

		...

		interconnects = <&gnoc MASTER_APPSS_PROC 3 &mnoc SLAVE_EBI1 3>;

	}