dt-bindings: mips: lantiq: Document Lantiq Xway PMU bindings

Document the Lantiq Xway SoC series Power Management Unit (PMU) bindings.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>