gpio: sysfs: remove the mockdev pointer from struct gpio_device

gpio: sysfs: remove the mockdev pointer from struct gpio_device

The usage of the mockdev pointer in struct gpio_device is limited to the
GPIO sysfs code. There's no reason to keep it in this top-level
structure. Create a separate structure containing the reference to the
GPIO device and the dummy class device that will be passed to
device_create_with_groups(). The !gdev->mockdev checks can be removed as
long as we make sure that all operations on the GPIO class are protected
with the sysfs lock.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250610-gpio-sysfs-chip-export-v1-6-a8c7aa4478b1@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>