pinctrl: mediatek: fix build for tristate changes

Export mtk_is_virt_gpio() for the case when
CONFIG_PINCTRL_MTK=y
CONFIG_PINCTRL_MTK_V2=y
CONFIG_PINCTRL_MTK_MOORE=y
CONFIG_PINCTRL_MTK_PARIS=m

to fix this build error:

ERROR: modpost: "mtk_is_virt_gpio" [drivers/pinctrl/mediatek/pinctrl-paris.ko] undefined!

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Sean Wang <sean.wang@kernel.org>
Cc: linux-mediatek@lists.infradead.org
Link: https://lore.kernel.org/r/d15827a3-d0c8-e231-9f61-8507b3d7be3a@infradead.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>