m68k/atari: EtherNAT - platform device and IRQ support code

Add platform device and interrupt definitions necessary for the EtherNAT
Ethernet/USB adapter for the Falcon extension port. EtherNAT interrupt
numbers are 139/140 so the max. interrupt number for Atari has to be
increased.

[Geert] Conditionalize platform device data structures

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