dt-bindings: mediatek: add bindings for MediaTek MT7622 SoC

This adds dt-binding documentation for MediaTek MT7622 SoC
which currently only includes basic items such as ARM CPU,
MediaTek SYSIRQ and UART.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>