ARM: tegra: Add DT binding for Tegra186 GPIO controllers

Tegra186 contains two separate but mostly similar GPIO controllers.
Register layout differs significantly from previous Tegra generations,
and so a new binding is required to describe them in device tree. This
patch adds that binding.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>