syscon: dt-bindings: Add binding doc for Broadcom iProc CDRU

Add binding document for Broadcom iProc's Chip Device Resource Unit
(CDRU). Multiple iProc based chips have this CDRU block that contains
miscellaneous registers for chip or device configurations. Start with
Stingray specific compatible string for the initial binding
Signed-off-by: NRay Jui <ray.jui@broadcom.com>
Reviewed-by: NScott Branden <scott.branden@broadcom.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

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

+Broadcom iProc Chip Device Resource Unit (CDRU)
+
+Various Broadcom iProc SoCs have a set of registers that provide various
+chip specific device and resource configurations. This node allows access to
+these CDRU registers via syscon.
+
+Required properties:
+- compatible: should contain:
+		"brcm,sr-cdru", "syscon" for Stingray
+- reg: base address and range of the CDRU registers
+
+Example:
+	cdru: syscon@6641d000 {
+		compatible = "brcm,sr-cdru", "syscon";
+		reg = <0 0x6641d000 0 0x400>;
+	};