ARM: kirkwood: convert to DT irqchip and clocksource

With recent support for true irqchip and clocksource drivers for Orion
SoCs, now make use of it on DT enabled Kirkwood boards.

This also introduces a new Kconfig option for legacy (non-DT) Kirkwood
where old code is moved out to and polishes DT board file a little bit.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>