dt-bindings: Add power domain info for NVIDIA PMC

Add power-domain binding documentation for the NVIDIA PMC driver in
order to support generic power-domains.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>