dt-bindings: bus: Add MIPS CDMM controller

It's a Common Device Memory Map controller embedded into the MIPS IP
cores, which dts node is supposed to have compatible and reg properties.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>