irqchip/armada-370-xp: Configure IPIs as standard interrupts

To introduce IPIs as standard interrupts to the Armada 370-XP
driver, let's allocate a completely separate irqdomain and
irqchip combo that lives parallel to the "standard" one.

This effectively should be modelled as a chained interrupt
controller, but the code is in such a state that it is
pretty hard to shoehorn, as it would require the rewrite
of the MSI layer as well.

Signed-off-by: Marc Zyngier <maz@kernel.org>