gpio: ixp4xx: Detect special machines by compatible

There are some special clock amendments for two machines
formerly detected by their machine_is() boardfile macro.

They are now migrated to device tree so use
of_machine_is_compatible() instead.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>