gpio: tegra: Support building driver as a loadable module

Support building driver as a loadable kernel module. This allows to
reduce size of a kernel zImage, which is important for some devices
since size of kernel partition may be limited and since some bootloader
variants have known problems in regards to the initrd placement if kernel
image is too big.

$ lsmod
Module                  Size  Used by
gpio_tegra             16384  27

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>