pinctrl: mediatek: add support for different types of IO pins

There are many pins in an SoC, and register usage may vary by pins.
This patch introduces a concept of "io type" and "io type group"
to mediatek pinctrl drivers. This can provide different pinconf
handlers implementation (eg: "bias-pull-up/down", "driving" and
"input-enable") for IO pins that belong to different types.

Signed-off-by: Sam Shih <sam.shih@mediatek.com>