gpio: tegra: Make lockdep class file-scoped

Commit b546be0db955 ("gpio: tegra: Get rid of all file scoped global
variables") moved all file scoped variables into the driver-private
structure to allow potentially multiple instances of the driver. The
change also included turning the lockdep class into a driver-private
field, which doesn't work and produces error messages such as this:

	[    0.142310] BUG: key ffff8000fb3f7ab0 not in .data!

Make the lockdep class file-scoped again to fix this issue.

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