dt-bindings: tegra186-gpio: Add Tegra186 specific prefix

Subsequent generations of Tegra, such as Tegra194, contain a completely
different set of GPIOs. In order to clarify that the Tegra186 defines
are indeed specific to Tegra186, change the prefix from TEGRA_ to
TEGRA186_.

Note that for now we need to keep the old definitions in place to avoid
breaking compilation in file that use this header. Once all users have
been converted to use the new defines, the old ones can be removed.

Also note that this is only a naming change and doesn't affect device
tree ABI.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>