dt-bindings: soc: tegra-pmc: Document core power domain

All NVIDIA Tegra SoCs have a core power domain where majority of hardware
blocks reside. Document the new core power domain properties.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>