ARM: integrator: convert to use irqchip_init

Now that versatile-fpga irqchip has IRQCHIP_DECLARE support, the interrupt
related initialization can be removed.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@arm.linux.org.uk>
Acked-by: Linus Walleij <linus.walleij@linaro.org>