dt-bindings: mips: Document mti,mips-cm

Add device tree binding documentation for MIPS Coherence Manager. This
component enables support for SMP by providing each processor in the
system with a uniform view of memory. The Coherence Manager is
responsible for establishing the global ordering of requests from all
elements of the system and sending the correct data back to the
requester.

Based on the work of Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>