gpio: Fix build warnings on undefined struct pinctrl_dev
This fixes the warnings:
* include/linux/gpio.h:254:11: warning: 'struct pinctrl_dev' declared
inside parameter list will not be visible outside of this definition
or declaration
* include/linux/gpio/driver.h:602:11: warning: 'struct pinctrl_dev'
declared inside parameter list will not be visible outside of this
definition or declaration