dt-bindings: clock: tegra-car: Document new clock sub-nodes

Document sub-nodes which describe Tegra SoC clocks that require a higher
voltage of the core power domain in order to operate properly on a higher
clock rates.  Each node contains a phandle to OPP table and power domain.

The root PLLs and system clocks don't have any specific device dedicated
to them, clock controller is in charge of managing power for them.

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