irqchip/irq-csky-mpintc: Remove unnecessary loop in interrupt handler

csky_mpintc_handler()
  ->handle_domain_irq()
    ->irq_exit()
      ->invoke_softirq()
        ->__do_softirq()
          ->local_irq_enable()

If new interrupt coming, it'll get into interrupt trap before return to
csky_mpintc_handler(). So there is no need loop in csky_mpintc_handler.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>