Document: DT: Add bindings for mediatek MT8173 SoC Platform

MT8173 is a 64-bit four core Soc.
And mt8173-evb is a evaluation board based on mt8173.
This commit add the devicetree binding document for
  mediatek MT8173 SoC
  MT8173 sysirq
  MT8173 uart

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