pinctrl: mediatek: avoid virtual gpio trying to set reg

for virtual gpios, they should not do reg setting and
should behave as expected for eint function.

Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
Signed-off-by: Hanks Chen <hanks.chen@mediatek.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Link: https://lore.kernel.org/r/1595503197-15246-4-git-send-email-hanks.chen@mediatek.com


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