pinctrl: mediatek: fix warning

Fix the following warning when CONFIG_PINCONF=n:

drivers/pinctrl/mediatek/pinctrl-mtk-common.c:35:36:
warning: ‘mtk_drive’ defined but not used [-Wunused-const-variable=]
 static const struct mtk_drive_desc mtk_drive[] = {
                                    ^~~~~~~~~

Signed-off-by: Fabien Parent <fparent@baylibre.com>