irqchip: mips-gic: Configure EIC when CPUs come online

Rather than configuring EIC mode for all CPUs during boot, configure it
locally on each when they come online. This will become important with
multi-cluster support, since clusters may be powered on & off (for
example via hotplug) and would lose the EIC configuration when powered
off.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>