gpio: Properly document parent data union

Suppress a warning in the html docs by documenting these fields separately.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/lkml/20211027220118.71a229ab@canb.auug.org.au/


Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>