dt-bindings: mips: add CPU bindings for MIPS architecture

Add the yaml binding for available CPUs in MIPS architecture.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@fungible.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>