After a long few months trying to figure this out...
It looks like the PL011 UART driver is unable to set a baudrate when booted via u-boot. My understanding of the code is poor but it looks like it only works for x86 BIOS/EFI booted systems.
This seems like something that might be useful for people running HAT's on raspberry pi's or other ARM boards.
Currently the baudrate can be set using u-boot by adding
init_baudrate="9600" to /boot/msdos/config.txt but it FreeBSD is unable to change it.
It would be nice to have baudrate setting implemented eventually... but at least now there is a ticket for other people having this problem.