gpio: mpc8xxx: ls1088a/ls1028a edge detection mode bug fixs.

On these boards, the irq_set_type must point one valid function pointer
that can correctly set both edge and falling edge.

Signed-off-by: Song Hui <hui.song_1@nxp.com>
Link: https://lore.kernel.org/r/20191122061839.24904-1-hui.song_1@nxp.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>