DOC: ARC: update arc status page

We support ARCv3 HS6x SMP systems now - let's update the
status in the documentation.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>