pinctrl: mediatek: Add support for pin configuration dump via debugfs.

Add support for pin configuration dump via catting
/sys/kernel/debug/pinctrl/$platform_dependent_path/pinconf-pins.
pinctrl framework had already support such dump. This patch implement the
operation function pointer to fullfill this dump.

Signed-off-by: Light Hsieh <light.hsieh@mediatek.com>
Link: https://lore.kernel.org/r/1579675994-7001-6-git-send-email-light.hsieh@mediatek.com
Acked-by: Sean Wang <sean.wang@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>