irqchip/bcm7038-l1: Implement irq_cpu_offline() callback

We did not implement an irq_cpu_offline callback for our irqchip, yet we
support setting a given IRQ's affinity. This resulted in interrupts
whose affinity mask included CPUs being taken offline not to work
correctly once the CPU had been put offline.

Fixes: 5f7f0317

 ("IRQCHIP: Add new driver for BCM7038-style level 1 interrupt controllers")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: jason@lakedaemon.net
Cc: marc.zyngier@arm.com
Cc: cernekee@gmail.com
Cc: jaedon.shin@gmail.com
Cc: ralf@linux-mips.org
Cc: justinpopo6@gmail.com
Link: http://lkml.kernel.org/r/1477948656-12966-2-git-send-email-f.fainelli@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>