gpio: tegra: don't auto-enable for COMPILE_TEST

I stumbled over a build error with COMPILE_TEST and CONFIG_OF
disabled:

drivers/gpio/gpio-tegra.c: In function 'tegra_gpio_probe':
drivers/gpio/gpio-tegra.c:603:9: error: 'struct gpio_chip' has no member named 'of_node'

The problem is that the newly added GPIO_TEGRA Kconfig symbol
does not have a dependency on CONFIG_OF. However, there is another
problem here as the driver gets enabled unconditionally whenever
COMPILE_TEST is set.

This fixes both problems, by making the symbol user-visible
when COMPILE_TEST is set and default-enabled for ARCH_TEGRA=y.

As a side-effect, it is now possible to compile-test a Tegra
kernel with GPIO support disabled, which is harmless.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 4dd4dd1d

 ("gpio: tegra: Allow compile test")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>