pinctrl: MediaTek: add pinctrl driver for MT7629 SoC

This patch adds pinctrl support for MT7629 SoC. The IO core found on
the SoC has the registers for pinctrl, pinconf and gpio mixed up in
the same register range. Hence the driver also implements the gpio
functionality through UCLASS_GPIO.

This also creates a common file as there might be other chips that use
the same binding and driver, then being a little more abstract could
help in the long run.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>