pinctrl: mediatek: improve Kconfig dependencies

Remove prompts to make all pinctrl cores to non-visible symbols and
make sure the target SoCs would be coupled with the corresponding
cores.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Acked-by: Sean Wang <sean.wang@kernel.org>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>