dt-bindings: gpio: Use Tegra186-specific include guard

Use a unique include guard for the Tegra186 GPIO DT bindings header to
avoid clashes with the DT bindings header for earlier chips.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20201127140852.123192-2-thierry.reding@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>