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>

drivers/gpio/Kconfig

@@ -595,7 +595,7 @@ config GPIO_TB10X
 	select OF_GPIO
 
 config GPIO_TEGRA
-	bool "NVIDIA Tegra GPIO support"
+	tristate "NVIDIA Tegra GPIO support"
 	default ARCH_TEGRA
 	depends on ARCH_TEGRA || COMPILE_TEST
 	depends on OF_GPIO

drivers/gpio/gpio-tegra.c

@@ -802,6 +802,7 @@ static const struct of_device_id tegra_gpio_of_match[] = {
 	{ .compatible = "nvidia,tegra20-gpio", .data = &tegra20_gpio_config },
 	{ },
 };
+MODULE_DEVICE_TABLE(of, tegra_gpio_of_match);
 
 static struct platform_driver tegra_gpio_driver = {
 	.driver = {
@@ -811,9 +812,11 @@ static struct platform_driver tegra_gpio_driver = {
 	},
 	.probe = tegra_gpio_probe,
 };
-
-static int __init tegra_gpio_init(void)
-{
-	return platform_driver_register(&tegra_gpio_driver);
-}
-subsys_initcall(tegra_gpio_init);
+module_platform_driver(tegra_gpio_driver);
+
+MODULE_DESCRIPTION("NVIDIA Tegra GPIO controller driver");
+MODULE_AUTHOR("Laxman Dewangan <ldewangan@nvidia.com>");
+MODULE_AUTHOR("Stephen Warren <swarren@nvidia.com>");
+MODULE_AUTHOR("Thierry Reding <treding@nvidia.com>");
+MODULE_AUTHOR("Erik Gilling <konkers@google.com>");
+MODULE_LICENSE("GPL v2");