m68k: Add common forward declaration for show_registers()

There are several forward declarations for show_registers() in C source
files.  Replace these by a single common forward declaration in
<asm/processor.h>, and include <asm/processor.h> where needed.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Link: https://lore.kernel.org/r/d5b6a7f9af3e82f0ccb67edac09d9ee45d457932.1657114791.git.geert@linux-m68k.org