m68k: atari: Make atari_platform_init() static

m68k: atari: Make atari_platform_init() static

When building with W=1:

    arch/m68k/atari/config.c:883:12: warning: no previous prototype for ‘atari_platform_init’ [-Wmissing-prototypes]
      883 | int __init atari_platform_init(void)
	  |            ^~~~~~~~~~~~~~~~~~~

Fix this by making atari_platform_init() static.
There was never a user outside arch/m68k/atari/config.c.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/417b46ec1945212c0b1fcc8b0f4074bae9a8b0b4.1694613528.git.geert@linux-m68k.org