irqchip/armada-370-xp: Only call ipi_resume() if IPI is available

IPI is available only on systems where the mpic controller does not have a
parent interrupt defined (e.g. on Armada XP). If a parent interrupt is
defined, inter-processor interrupts are handled by an interrupt controller
higher in the hierarchy (most probably a parent GIC).

Only call ipi_resume() on systems where IPI is available in the mpic
controller.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>

[ refactored a little and changed commit message ]