m68k: mm: Unify Motorola MMU page setup

Seeing how there are 5 copies of this magic code, one of which is
unexplainably different, unify and document things.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Tested-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20200131125403.597688427@infradead.org


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