gpio: lpc18xx: 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: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>