MAINTAINERS: update entry for Mediatek pin controller

Add all source files under drivers/pinctrl/mediatek for the entry and
change the address to a permanent one since I have a personal leave
over the next few months and the address would be suspended for a while.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>