pinctrl: mediatek: convert to arch_initcall

Move pinctrl initialization earlier in boot so that real devices can find
their pctldev without probe deferring.

Note: We don't change mt6397 probe order in this patch, since MT6397 is mfd
PMIC, which depends on pwrap on main AP to work. Since pmic-wrap itself
is module_platform_driver, we keep it as module_init.  A later patch
will convert both pmic-wrap, and all functions of the MT6397 mfd to
arch_initcall.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>