gpio: mpc8xxx: Convert to platform device interface.

This way we do not need to transverse the device tree manually.

Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>