While not strictly required for normal operation setting the GPIO parent
device allows the GPIO framework to generate more verbose debug output for
the GPIO chip.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>