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