gpio: mcux: Revert to older GPIO device names as were used for Kinetis.

These are more consistent with naming used by other ports (uppercase,
short), and some existing software relies on them to be exactly those.
This change is a follow up to the discussion on the Zephyr mailing
list, calling to establish consistent naming conventions for Zephyr
devices, and is a small step in that direction.

Change-Id: I013b0505b579c6337aeb6fbef2423216ca6cf046
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>