When building with W=1:

    arch/m68k/mm/motorola.c:414:13: warning: no previous prototype for ‘paging_init’ [-Wmissing-prototypes]
      414 | void __init paging_init(void)
	  |             ^~~~~~~~~~~
    arch/m68k/mm/sun3mmu.c:36:13: warning: no previous prototype for ‘paging_init’ [-Wmissing-prototypes]
       36 | void __init paging_init(void)
	  |             ^~~~~~~~~~~

Fix this by consolidating the multiple prototypes into the common
<asm/pgtable.h>.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/1b03fde54f205e972e19959b8e335022205d538c.1694613528.git.geert@linux-m68k.org