m68k: coldfire: ensure gpio prototypes visible

When building with W=1:

      CC      arch/m68k/coldfire/gpio.o
    arch/m68k/coldfire/gpio.c:19:5: warning: no previous prototype for ‘__mcfgpio_get_value’ [-Wmissing-prototypes]
     int __mcfgpio_get_value(unsigned gpio)
         ^~~~~~~~~~~~~~~~~~~
    arch/m68k/coldfire/gpio.c:25:6: warning: no previous prototype for ‘__mcfgpio_set_value’ [-Wmissing-prototypes]
     void __mcfgpio_set_value(unsigned gpio, int value)
          ^~~~~~~~~~~~~~~~~~~
    arch/m68k/coldfire/gpio.c:50:5: warning: no previous prototype for ‘__mcfgpio_direction_input’ [-Wmissing-prototypes]
     int __mcfgpio_direction_input(unsigned gpio)
         ^~~~~~~~~~~~~~~~~~~~~~~~~
    arch/m68k/coldfire/gpio.c:65:5: warning: no previous prototype for ‘__mcfgpio_direction_output’ [-Wmissing-prototypes]
     int __mcfgpio_direction_output(unsigned gpio, int value)
         ^~~~~~~~~~~~~~~~~~~~~~~~~~
    arch/m68k/coldfire/gpio.c:96:5: warning: no previous prototype for ‘__mcfgpio_request’ [-Wmissing-prototypes]
     int __mcfgpio_request(unsigned gpio)
         ^~~~~~~~~~~~~~~~~
    arch/m68k/coldfire/gpio.c:102:6: warning: no previous prototype for ‘__mcfgpio_free’ [-Wmissing-prototypes]
     void __mcfgpio_free(unsigned gpio)
          ^~~~~~~~~~~~~~

The local m68k asm version of gpio.h has prototypes for all of these,
but they are not always visible depending on the config options enabled.
Move the prototypes so they are always visible.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>