irqchip: support cascaded VICs

This adds support for a VIC to be cascaded off another IRQ.
On the Integrator/AP logical module IM-PD1 there is a VIC
cascaded off the central FPGA IRQ controller so this is
needed for that to work out.

In order for the plug-in board to be able to register all
the devices with their IRQs relative to the offset of the
base obtained for the cascaded VIC, the base IRQ number
is passed back to the caller.

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