m68k/atari: EtherNEC - add platform device support

Add platform device for the Atari ROM port ethernet adapter, EtherNEC.
This platform device will be used by the ne.c driver.

[Geert] Conditionalize platform device data structures

Signed-off-by: Michael Schmitz <schmitz@debian.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>