irqchip: versatile-fpga: Add IRQCHIP_DECLARE support

Add support for initialization using IRQCHIP_DECLARE. This also requires
that the controller initialization set the handle_irq function pointer
itself when it is a primary controller.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Acked-by: Linus Walleij <linus.walleij@linaro.org>