The latest AMD motherboard to be supported by upstream Coreboot is the Jetway NF81-T56N-LF. The Jetway NF81-T56N-LF is an embedded IPC motherboard that bears a dual-core 1.6GHz AMD Fusion G-T56N APU with two DDR3 memory sockets, an AMD A55E "Hudson" chipset, and other goodies. Those wanting more information on this new open-source friendly motherboard can find details at jetway.com.tw.
The support for the Jetway NF81-T56N-LF landed in Coreboot on Saturday with this Git commit and then the actual mainboard support. Those curious about a full list of known supported motherboards by Coreboot can find a listing at Coreboot.org.