ARC: smp: irqchip: handle IPI as percpu irq like timer

The reason this was not done so far was lack of genuine IPI_IRQ for
ARC700, as we don't have a SMP version of core yet (which might change
soon thx to EZChip). Nevertheles to increase the build coverage, we
need to allow CONFIG_SMP for ARC700 and still be able to run it on a
UP platform (nsim or AXS101) with a UP Device Tree (SMP-on-UP)

The build itself requires some define for IPI_IRQ and even a dummy
value is fine since that code won't run anyways.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>