pinctrl: mediatek: extend struct mtk_pin_desc to pinctrl-mtk-common-v2.c

This patch introduces a data structure mtk_pin_desc, which is used to
provide information per pin characteristic such as driving current,
eint number and a driving index, that is used to lookup table describing
the details about the groups of driving current by which the pin is able
to adjust the driving strength so that the driver could get the
appropriate driving group when calls .pin_config_get()/set().

Signed-off-by: Ryder.Lee <ryder.lee@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>