gpiolib: fix sysfs when cdev is not selected

In gpiochip_setup_dev() the call to gpiolib_cdev_register() indirectly
calls device_add().  This is still required for the sysfs even when
CONFIG_GPIO_CDEV is not selected in the build.

Replace the stubbed functions in gpiolib-cdev.h with macros in gpiolib.c
that perform the required device_add() and device_del() when
CONFIG_GPIO_CDEV is not selected.

Fixes: d143493c

 (gpiolib: make cdev a build option)
Reported-by: Nicolas Schichan <nschichan@freebox.fr>
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Tested-by: Nicolas Schichan <nschichan@freebox.fr>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>