m68k: fix recursive dependency in Kconfig

We had a recursive dependency between MMU_MOTOROLA and MMU_SUN3
Fix it by dropping the unused dependencies on MMU_MOTOROLA.

MMU_MOTOROLA is set to y only using select so any dependencies
are anyway ignored.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Roman Zippel <zippel@linux-m68k.org>