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>