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>
(cherry picked from commit e33b67523f556aa7ddb09f1c7fa4de5c080670c9)
Signed-off-by: Alex Shi <alex.shi@linaro.org>