arm64: dts: mediatek: add basic mt7986 support

Add basic chip support for Mediatek mt7986, include
basic uart nodes, rng node and watchdog node.

Add cpu node, timer node, gic node, psci and reserved-memory node
for ARM Trusted Firmware.

Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Link: https://lore.kernel.org/r/20211122123222.8016-3-sam.shih@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>