hw/intc/arm_gic: Add external IRQ lines for VIRQ and VFIQ

Augment the GIC's QOM device interface by adding two
new sets of sysbus IRQ lines, to signal VIRQ and VFIQ to
each CPU.

We never use these, but it's helpful to keep the v2-and-earlier
GIC's external interface in line with that of the GICv3 to
avoid board code having to add extra code conditional on which
version of the GIC is in use.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1483977924-14522-3-git-send-email-peter.maydell@linaro.org