irqchip: versatile FPGA: support cascaded interrupts from DT

The Versatile FPGA interrupt controller supports cascading interrupts,
i.e. that its output is connected to the input of another interrupt
controller. This makes it possible to pass a parent interrupt from
the device tree and print it in the boot log if applicable.

Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>