I'm successfully using U-boot with Coreboot on the Rangeley platform
with the attached dts file. I'm not sure if everything in it is needed
and/or correct, but it works fine.
serial1.dtsi was needed so that the second UART is used as stdout.
Although, this question looks like more relevant to the U-boot mailing
list than the Coreboot one.
coreboot mailing list: email@example.com