m68k: Add asm/config.h

To avoid 'warning: no previous prototype for' errors, declare all the
parse_bootinfo and config function prototypes into asm/config.h and
include it in arch/m68k/kernel/setup_mm.c and arch/m68k/*/config.c.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Link: https://lore.kernel.org/r/20220121200738.2577697-2-laurent@vivier.eu


Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>