UART IP in Vybrid SoC can run on much faster speed than it is normally required. So there is no sense to specify it. We can compute theoretical maximum from the clock chain.
Let's say IPS_bus_clock which is used to clock UART module is 66MHz. Baud rate is divided at least by 16 times due to bit filtering.
UART baud rate = UART module clock / (16 × (SBR[12:0] + BRFD)).
So theoretical maximum is 66.6MHz / 16 = 4.1625 MHz. Real maximal value would have to be simulated and then tested.