gpio: mpc8xxx: remove __initdata annotation for mpc8xxx_gpio_ids[]

Since commit 98686d9a ("gpio: mpc8xxx: Convert to platform device
interface"), we get the following section mismatch warning. Remove the
__initdata annotation to fix it.
  WARNING: vmlinux.o(.data+0xbc28): Section mismatch in reference from the variable mpc8xxx_plat_driver to the variable .init.data:mpc8xxx_gpio_ids
  The variable mpc8xxx_plat_driver references
  the variable __initdata mpc8xxx_gpio_ids
  If the reference is valid then annotate the
  variable with __init* or __refdata (see linux/init.h) or name the variable:
  *_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>