dm: gpio: Add a comment about not copying some drivers

These three drivers all use U_BOOT_DEVICE rather than device tree to
create devices, so have to do manual allocation of platform data. This is
not true for new platforms.

Add a more explicit comment so that people do not copy this approach with
new boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Adam Ford <aford173@gmail.com>