arm64: dts: mt7622: add timer, CCI-400 and PMU nodes

Add device tree entries for timer, ARM CCI-400 and its PMU.
Otherwise, we add a cortex-a53-pmu node to enable hw perfevents.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>