arm64: dts: mt7622: add basic nodes to the mt7622.dtsi file

add basic nodes into the mt7622.dtsi for the system
bring-up which includes ARM CPU, GIC, timer, MediaTek
UART, SYSIRQ and one reserved memory region for ATF.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>