gpio: provide stubs for devres gpio functions

commit 6b3d8145dcfdbbb43f13544e16f44f4574f941dd
"gpiolib: make GPIO_DEVRES depend on GPIOLIB"
breaks builds when device drivers are using devm_gpio*
devres functions without enabling GPIOLIB, relying on
the devres code to be compiled anyway.

Provide stubs so that we get these if we're using the
devres functions without GPIOLIB.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>