powerpc/virtex: Use generic xilinx irqchip driver

The Xilinx interrupt controller driver is now available in drivers/irqchip.
Switch to using that driver.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
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>