MFC r355572:

Allow baud rates of 1,228,800 and 1,843,200 on CP2101/2/3 usb-serial adapters.

The datasheets for these chips claim the maximum is 921,600, but testing
shows these two higher rates also work (but no rates above 921,600 other
than these two work; these represent dividing the base buad clock by 3 and 2
respectively).