drm/tegra: sor: Reimplement pad clock

The current implementation of the pad clock isn't quite correct. This
has the side-effect of being incompatible with the implementation for
Tegra186 (provided by the BPMP) and therefore would require a massive
change to the driver to cope with the differences. Instead, simply do
what Tegra186 does and add some code to fallback to the old behaviour
for existing device trees.

Signed-off-by: Thierry Reding <treding@nvidia.com>