arm64: dts: rockchip: Enable UART6 on rock-5b

Enable UART lines on Radxa ROCK 5 Model B M.2 Key E.

Signed-off-by: Tamás Szűcs <szucst@iit.uni-miskolc.hu>
Link: https://lore.kernel.org/r/20231013215208.81345-1-szucst@iit.uni-miskolc.hu


Signed-off-by: Heiko Stuebner <heiko@sntech.de>