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: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>