m68k/io: Add missing ioremap define guards, fix typo

  - Add missing define guard for ioremap_wt(),
  - Move ARCH_HAS_IOREMAP_WT from <asm/io_mm.h> to <asm/kmap.h>, as it
    is applicable to Coldfire with MMU, too,
  - Fix typo s/ioremap_fillcache/ioremap_fullcache/,
  - Add define guard for iounmap() for consistency with other
    architectures.

Fixes: 9746882f ("m68k: group io mapping definitions and functions")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>