The mcbin (and likely others) have a nonstandard uart clock. This means that the earlycon programming will incorrectly set the baud rate if it is specified. The way around this is to tell the kernel to continue using the preprogrammed baud rate. This is done by setting the baud to 0.
Our drivers (uart_dev_ns8250) do respect zero, but SPCR would error. Let's not error.
(Also, looks like Marvell has made a mess with access widths *on an 8250/16550*… W T F)