The UARTs themselves were already present, just not configured under any
alias.
Follow [ArduPilot UART naming for this
board](https://ardupilot.org/plane/docs/common-pixracer-pro.html#default-uart-order),
out of necessity: it's the only combination that makes sense and is
understood by QGroundControl.
(For example, I've attempted configuring the additional UARTs as `_EXT1`
and `_EXT2`, but only `_EXT2` was understood by QGC.)
Trying to actually use the FrSky telemetry port for anything without
configuring FrSky telemetry on a different port may be impossible without
modifying the board's `init/rc.board_extras`.
(Should that script be modified to check if the port is used for
anything else?)
Skip naming the RC input port.
Fixes#21455.
1a3cdecb39