Add support for the new smp_twd runtime registration interface
to the imx6q platforms, and remove the old compile-time support.

The imx6q DTS file is updated to match the TWD DT documentation.
Also present in this patch a DTS fix to the timer interrupt routing
(the PPI connection uses bits [15:8]) and trigger (rising edge).
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>