dt-bindings: gpio: tegra186: Convert to json-schema

Convert the Tegra186 GPIO controller device tree bindings from free-form
text format to json-schema.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>