gpio: fix abi regression in sysfs

We started to assign the gpio_device as parent for the sysfs
but this changes the expected layout of sysfs. Restore the
previous behaviour.

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>