ARM: imx6: Allow GPC interrupts affinity to be changed

While converting the GPC code to a stacked irqchip, we lost the
possibility to change the CPU affinity of an interrupt routed
through the GPC.

This patch restore the expected behaviour by forwarding the
affinity setup to the underlying irqchip (GIC).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>