gpio: mxs: remove __init annotation

Building with an old toolchain, I ran into this warning:

WARNING: vmlinux.o(.text+0x63eef0): Section mismatch in reference
  from the function mxs_gpio_probe() to the function
  .init.text:mxs_gpio_init_gc()

Clearly the annotation is wrong, since the function is called from the
non-init probe, so let's remove it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>