irqchip/xilinx: Rename get_irq to xintc_get_irq

Now that the driver is generic and used by multiple archs,
get_irq is too generic.

Rename get_irq to xintc_get_irq to avoid any conflicts

Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>