arm64: dts: realtek: Add RTD1619 SoC and Realtek Mjolnir EVB

Add Device Trees for Realtek RTD1619 SoC family, RTD1619 SoC and
Realtek Mjolnir EVB.

Signed-off-by: James Tai <james.tai@realtek.com>
[AF: Renamed r-bus node, modified UART comments, style cleanups]
Signed-off-by: Andreas Färber <afaerber@suse.de>