ARM: gic: add OF based initialization

This adds ARM gic interrupt controller initialization using device tree
data.

The initialization function is intended to be called by of_irq_init
function like this:

const static struct of_device_id irq_match[] = {
	{ .compatible = "arm,cortex-a9-gic", .data = gic_of_init, },
	{}
};

static void __init init_irqs(void)
{
	of_irq_init(irq_match);
}

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Reviewed-by: Jamie Iles <jamie@jamieiles.com>
Tested-by: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>