ARC: Remove claims that we don't support HS2

Updated Readme document:
* Remove claims that HS2 is no supported, because that has been fixed
* Standardize naming of Digilent cable: remove any mention "probe" and
  "dongle" with cable, because that is how it is called on Digilent
  site.

Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>