pinctrl: mediatek: do not probe gpio driver if not enabled

The mtk pinctrl driver is a combination driver with support for both
pinctrl and gpio. When this driver is used in SPL, gpio support may not be
enabled, and this will result in a compilation error.

To fix this, macros are added to make sure gpio related code will only be
compiled when gpio support is enabled.

Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>