irqchip: irq-armada-370-xp: Do the set_smp_cross_call() in the driver

Instead of having the SoC code in arch/arm/mach-mvebu/platsmp.c do the
set_smp_cross_call() to register the IPI-triggering function, it makes
more sense to do exactly what the GIC driver is doing: let the irqchip
driver do it. This way, it avoids having to expose the
armada_mpic_send_doorbell() function between the irqchip driver and
the SoC code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1397483648-26611-5-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>