ARM: dts: mt8173: support arm64 cpuidle-dt

This patch adds an idle-states node to describe the mt8173 idle states and
also adds references to the idle-states node in all CPU nodes.

Signed-off-by: Howard Chen <howard.chen@linaro.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>