irqchip: add C-SKY APB bus interrupt controller

The driver is for C-SKY APB bus interrupt controller. It's a simple
interrupt controller which use pending reg to detect the irq and use
enable/disable reg to mask/unmask interrupt sources.

A lot of SOCs based on C-SKY CPU use the interrupt controller as root
controller.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>