gpio: mpc8xxx: Add platform device to gpiochip->parent

Dear Linus Walleij,

In old kernels, some APIs still try to use parent->of_node from struct gpio_chip,
and it could be resulted in kernel panic because parent is NULL. Adding platform
device to gpiochip->parent can fix this problem.

Signed-off-by: Johnson Chen <johnsonch.chen@moxa.com>
Link: https://patchwork.kernel.org/patch/11234609
Link: https://lore.kernel.org/r/HK0PR01MB3521489269F76467DFD7843FFA450@HK0PR01MB3521.apcprd01.prod.exchangelabs.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>