This is the current status.

soda> BTW, I encountered next problem. :-)
soda> Probably this is spl related. ARC's spl handling should be fixed anyway,
soda> so I'm trying to fix it, now.

arc/arc/machdep.c incorrectly used TLB 0,1,2 which are already reserved
by locore_r4000.S. this bug was fixed, now.

Now, kernel successfully mounts nfs root, and exec init, and init 
fork & exec /bin/sh, but sh dumps core.
Hmm, let's fix next problem.
--
soda