mips: txx9: change GENERIC_GPIO to GPIOLIB

The GPIO driver for txx9 requires gpiolib, so this is more accurate and
let us get rid of one mention of GENERIC_GPIO which is due to disappear.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

arch/mips/txx9/generic/setup.c

@@ -118,7 +118,7 @@ EXPORT_SYMBOL(clk_put);
 
 /* GPIO support */
 
-#ifdef CONFIG_GENERIC_GPIO
+#ifdef CONFIG_GPIOLIB
 int gpio_to_irq(unsigned gpio)
 {
 	return -EINVAL;