irqchip/mmp: Do not call irq_set_default_host() on DT platforms

Using a default domain on DT platforms is unnecessary, as the firmware
tables describe the full topology, and nothing is implicit.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
[maz: wrote an actual changelog]
Signed-off-by: Marc Zyngier <maz@kernel.org>