ARM: imx6: Warn when an old DT is detected

Now that the GPC has been converted to be a full blown irqchip
(and not a mole on the side of the GIC), booting a new kernel
with an old DT is likely to result in a rough ride for the user.

This patch makes sure such a situation is promptly detected and
the user made aware that a DT update is in order.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>