Where to modify the rpi5 kernel dts to enable the extra UART in pinout?

[tonyho]

I see there's a cm2712-rpi-5-b.dtb file but I cannot find the corresponding dts file using find -name cm2712-rpi-5-b.dts*.

I want to expose a TTL in pinout from kernel to be used for my Bluetooth controller, for example the GPIO4 and GPIO5 as UART2 RX/TX. But don't know how to enable it.

As in other boards like the amlogic and rockchip based ones, it can be archived by modify the dts file.

Any tips?