irqchip: mips-gic: Handle pending interrupts once in __gic_irq_dispatch()

When an interrupt occurs __gic_irq_dispatch() continuously reads local
and shared pending registers until all is serviced before
returning. The problem with that is that it could introduce a long
delay before returning if a piece of hardware keeps triggering while
in one of these loops.

To ensure fairness and priority doesn't get skewed a lot, read local
and shared pending registers once to service each pending IRQ once.
If another interupt triggers while servicing the current ones, then we
shall re-enter the handler after we return.

Signed-off-by: Qais Yousef <qais.yousef@imgtec.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: <linux-mips@linux-mips.org>
Link: http://lkml.kernel.org/r/1421668289-828-1-git-send-email-qais.yousef@imgtec.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>