dt-bindings: tegra: Add documentation for nvidia,tegra194-pmc

The Tegra194 power management controller has one additional register
aperture to be specified in the device tree node.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>