gpio: mpc8xxx: add ls1088a platform special function

ls1028a and ls1088a platform share common special function.
The gpio hardware what they use is the same version.

Signed-off-by: Song Hui <hui.song_1@nxp.com>
Link: https://lore.kernel.org/r/20190808101628.36782-3-hui.song_1@nxp.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>