arm64: dts: Add mediatek MT8173 SoC and evaluation board dts and Makefile

Add device tree support for MT8173 SoC and evaluation board based on it.

Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Acked-by: Arnd Bergmann <arnd at arndb.de>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>