pinctrl: mediatek: add OF dependency to MT6397

X86_64 allmodconfig screams like so:
warning: (PINCTRL_MT6397) selects PINCTRL_MTK_COMMON
which has unmet direct dependencies
(PINCTRL && (ARCH_MEDIATEK || COMPILE_TEST) && OF)

So add OF to dependencies to shut up this warning.

Cc: Hongzhou Yang <hongzhou.yang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>