soc/tegra: pmc: Use TEGRA186_ prefix for GPIO names

The new prefix allows the GPIOs to be uniquely identified on a per-chip
basis, which makes it easier to distinguish Tegra186 specific GPIOs from
those of later chips such as Tegra194 which supports a very different
set of GPIOs.

Signed-off-by: Thierry Reding <treding@nvidia.com>