mips: ar7: include linux/gpio/driver.h

mips: ar7: include linux/gpio/driver.h

The change to remove the implicit gpio/driver.h include was done
after fixing all the other users, but the ar7 file still needs
the same change.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: 21d9526d13b5 ("gpiolib: Make the legacy <linux/gpio.h> consumer-only")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>