I should add a couple of remarks.
1) The patched lines  to machdep.c that read
  + #if 0
  +
  + #include <arc/arc/vgatable.h>

should be changed to
  + #if NVGA_ISA > 0
  +
  + #include <arc/arc/vgatable.h>
2. I set osloadoptions flag to 's' serial console
so 'printf' statement appear display at serial console.

3. I looked at the bitmapped VGA wscons driver
mentioned by David Brownlee and it too
uses the same the 'vgaprob()' function, as vga
text driver which assumes the video chip
is in 'text' mode.