pinctrl/mediatek: Use "" for local includes

In file included from drivers/pinctrl/mediatek/pinctrl-mt6397.c:23:0:
drivers/pinctrl/mediatek/pinctrl-mtk-mt6397.h:19:32: fatal error: pinctrl-mtk-common.h: No such file or directory
 #include <pinctrl-mtk-common.h>