mips: xilfpga: Add device tree files

Mostly the same as the Kernel upstream device tree file except for

- alias for the serial console node
- ethernet node as the ethernet stuff isn't upstream on kernel.org yet
- uart clock-frequency passed directly in the node

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>