m68k/atari: use dedicated irq_chip for timer D interrupts

Add a special irq_chip for the Atari MFP timer D interrupt,
which is used as a polling timer for EtherNEC and NetUSBee

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