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

Add binding document for Broadcom iProc's Multi-Host Bridge (MHB)
block
Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/brcm,iproc-mhb.txt

+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>;
+	};