pinctrl: mediatek: extend eint build to pinctrl-mtk-common-v2.c

Almost all MediaTek SoCs apply the exact same logic to build eint, so move
the common functions into pinctrl-mtk-common-v2.c to allow each new pinctrl
driver to reuse them. Also, add a protection checker on hw->soc->eint_hw to
avoid invalid memory access when there's certain SoC not to define its
eint_hw properly in the code flow.

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