irqchip: add C-SKY SMP interrupt controller

The driver is for C-SKY SMP interrupt controller. It support 16
soft-irqs, 16 private-irqs, and 992 max external-irqs, a total of
1024 interrupts.

C-SKY CPU 807/810/860 SMP/non-SMP could use it.

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