staging: iio: Augment TODO file with GPIO work item

To make sure that these drivers do not leave staging before they
are properly converted to use the new GPIO descriptor API,
augment the TODO file with this work item.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>