dt: Document Tegra20/30 pinctrl binding

Define a new binding for the Tegra pin controller, which is capable of
defining all aspects of desired pin multiplexing and pin configuration.
This is all based on the new common pinctrl bindings.

Add Tegra30 binding based on Tegra20 binding.

Add some basic stuff that was missing before:
* How many and what reg property entries must be provided.
* An example.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>