pinctrl: apple: Implement GPIO func check callback

pinctrl: apple: Implement GPIO func check callback

This function will check if the indicated function enumerator
i.e. the value set into bits 5 and 6 of the pin configuration
register, is zero and thus corresponds to the GPIO mode of the
pin.

This may be necessary to know since the pin controller is
flagged as "strict": once you set a pin *explicitly* to GPIO
mode using function 0, the core is unaware that this is
actually the GPIO mode, and clients may be denied to
retrieve the pin as a GPIO.

Currently none of the in-kernel device trees sets any pin to
GPIO mode, but this may happen any day.

Reviewed-by: Sven Peter <sven@kernel.org>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>