dt-bindings: Add Tegra PMC pad configuration bindings

Document the PMC pinctrl bindings for pad power state and signaling
voltage configuration. Both nvidia,tegra186-pmc.txt and
nvidia,tegra20-pmc.txt are modified as they both cover SoC generations
for which these bindings apply.

Add a header defining Tegra PMC pad voltage configurations.

Signed-off-by: Aapo Vienamo <avienamo@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>