gpio: tegra: Drop exporting static functions

Both tegra_gpio_enable() and tegra_gpio_disable() are static functions, it does
not make sense to export them.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>