Thanks ShipingW_81 !
So far I understand it, max baud rate is not defined by the max frequency of the APB bus.
There must be other limitations in the hardware.
MB9B410T does have a max frequency of 72 MHz.
But with a 72 MHz frequency, the minimum value of BGR1/0 register, that is mentioned by the datasheet (Table 6-1 Reload Values and Baud Rates) is 8.
Hence the max bit rate is:
b = (72000000) / (8 + 1) = 8 Mbps according formel (1)
Conclusion, for all microcontrollers of the FM4 family the max baud rate is 8 Mbps.