gpio: mt7621: add OF_GPIO dependency

Compile-testing the driver fails unless OF_GPIO is enabled:

drivers/gpio/gpio-mt7621.c: In function 'mediatek_gpio_bank_probe':
drivers/gpio/gpio-mt7621.c:228:10: error: 'struct gpio_chip' has no member named 'of_node'

Fixes: 4ba9c3af ("gpio: mt7621: Add a driver for MT7621")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>