irqchip: mips-gic: Make IPI bitmaps static

We have 2 bitmaps used to keep track of interrupts dedicated to IPIs in
the MIPS GIC irqchip driver. These bitmaps are only used from the one
compilation unit of that driver, and so can be made static. Do so in
order to avoid polluting the symbol table & global namespace.

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>