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

The highbank DTS file is updated to match the TWD DT documentation
and fixes the timer trigger (rising edge).
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>