gpio: mpc8xxx: add ls1088a platform gpio node DT binding description

ls1088a and ls1028a platform share common gpio node.

Signed-off-by: Song Hui <hui.song_1@nxp.com>
Link: https://lore.kernel.org/r/20190808101628.36782-1-hui.song_1@nxp.com


Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>