mac68k: rework SWIM platform device

Adjust the platform device code to conform with the code style used in the
rest of this patch series. No need to name resources nor to register
devices which are not applicable.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>