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>

drivers/gpio/TODO

@@ -99,6 +99,10 @@ similar and probe a proper driver in the gpiolib subsystem.
 In some cases it makes sense to create a GPIO chip from the local driver
 for a few GPIOs. Those should stay where they are.
 
+At the same time it makes sense to get rid of code duplication in existing or
+new coming drivers. For example, gpio-ml-ioh should be incorporated into
+gpio-pch. In similar way gpio-intel-mid into gpio-pxa.
+
 
 Generic MMIO GPIO