m68k: atari: Make atari_stram_map_pages() static

m68k: atari: Make atari_stram_map_pages() static

When building with W=1:

    arch/m68k/atari/stram.c:118:12: warning: no previous prototype for ‘atari_stram_map_pages’ [-Wmissing-prototypes]
      118 | int __init atari_stram_map_pages(void)
	  |            ^~~~~~~~~~~~~~~~~~~~~

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

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/fa86b6d7e54d96ac52f574ed00f96e339f46990f.1694613528.git.geert@linux-m68k.org