m68k: mvme16x: Add and use "mvme16x.h"

m68k: mvme16x: Add and use "mvme16x.h"

When building with W=1:

    arch/m68k/mvme16x/config.c:208:6: warning: no previous prototype for ‘mvme16x_cons_write’ [-Wmissing-prototypes]
      208 | void mvme16x_cons_write(struct console *co, const char *str, unsigned count)
	  |      ^~~~~~~~~~~~~~~~~~

Fix this by introducing a new header file "mvme16x.h" for holding the
prototypes of functions implemented in arch/m68k/mvme16x/.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/6200cc3b26fad215c4524748af04692e38c5ecd2.1694613528.git.geert@linux-m68k.org