pinctrl: mediatek: include chained_irq.h header

Some randconfig builds fail in the mtk-eint driver:

drivers/pinctrl/mediatek/mtk-eint.c: In function 'mtk_eint_irq_handler':
drivers/pinctrl/mediatek/mtk-eint.c:324:2: error: implicit declaration of function 'chained_irq_enter'; did you mean 'rcu_irq_enter'? [-Werror=implicit-function-declaration]
  chained_irq_enter(chip, desc);
  ^~~~~~~~~~~~~~~~~
  rcu_irq_enter
drivers/pinctrl/mediatek/mtk-eint.c:367:2: error: implicit declaration of function 'chained_irq_exit'; did you mean 'rcu_irq_exit'? [-Werror=implicit-function-declaration]

The functions are declared in linux/irqchip/chained_irq.h, and including
that header makes it build in all configurations.

Fixes: e46df235

 ("pinctrl: mediatek: refactor EINT related code for all MediaTek pinctrl can fit")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>