gpio: Extend TODO to cover code duplication avoidance

It appears at least two drivers has a lot of duplication code in
GPIO subsystem. To avoid adding more and get rid of existing duplication
extend TODO.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>