nios2: rebase nios2-generic board to 3c120 reference design

Though nios2-generic board meant to be a template, it is helpful
to be able to test on a real hardware. As the nios2 linux is
developed and tested on a 3c120 FPGA based Golden Hardware Reference
Design, it makes sense to rebase nios2-generic on this FPGA design.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>