dt-bindings: Add power domains to Tegra BPMP firmware

The Tegra186 BPMP is also a provider of power domains. Enhance the
device tree binding to describe this.

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