gpio: return NULL from gpiod_get_optional when GPIOLIB is disabled

Given the intent behind gpiod_get_optional() and friends it does not make
sense to return -ENOSYS when GPIOLIB is disabled: the driver is expected to
work just fine without gpio so let's behave as if gpio was not found.
Otherwise we have to special-case -ENOSYS in drivers.

Note that there was objection that someone might forget to enable GPIOLIB
when dealing with a platform that has device that actually specifies
optional gpio and we'll break it. I find this unconvincing as that would
have to be the *only GPIO* in the system, which is extremely unlikely.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>