gpio: mpc8xxx: Remove unused of_gpio.h inclusion

of_gpio.h shouldn't be used in GPIO drivers. Remove it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>