syscon: dt-bindings: Add binding document for iProc MHB block

Broadcom iProc Multi Host Bridge (MHB)

Certain Broadcom iProc SoCs have a multi host bridge (MHB) block that controls

the connection and configuration of 1) internal PCIe serdes; 2) PCIe endpoint

interface; 3) access to the Nitro (network processing) engine

This node allows access to these MHB registers via syscon.

Required properties:

- compatible: should contain:

		"brcm,sr-mhb", "syscon" for Stingray

- reg: base address and range of the MHB registers

Example:

	mhb: syscon@60401000 {

		compatible = "brcm,sr-mhb", "syscon";

		reg = <0 0x60401000 0 0x38c>;

	};