dt: Move Tegra20 pin mux binding into new pinctrl directory

This places the file in the new location for all pin controller bindings.

Also, rename the file using the full compatible value for easier
avoidance of conflicts between multiple bindings.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>