The purpose of this file was to simply detect the UART speed before attaching the actual ns8250 driver so that we don't have to specify the UART speed in DTS files.
However, OpenWRT DTS files specify ns16550a as a compatible string in their DTS files and this makes the original ns8250 driver attach to the device. So we would have to edit the DTS files anyway and since this is only the case for MT7621 and MT7628/MT7688 for now, it's better to just add the clock-frequency property to those (UART is always clocked by the same clock in both these SoCs, so that's fine) instead of having a separate driver and still having to change the DTS files.