MAINTAINERS: Add a missing file to the INTEL GPIO section

When gpio-tangier was split the new born headers had been missed
in the MAINTAINERS. Add it there.

Fixes: d2c19e89 ("gpio: tangier: Introduce Intel Tangier GPIO driver")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>