According to the datasheet (https://de.scribd.com/document/948890791/MT7988A-Datasheet) the supported baudrate seems to be between 300baud and 3Mbaud. However cannot tell anything about support status. Neither do I have this hw, nor did I ever dig into this soc.
Starting with device tree overlays might be a good start or check the whole device tree in general for the uart nodes if present.