arc: emsdp: docs: Prefer Degilent over Opella-XD

Back in the day on early board samples built-in Digilent JTAG probe
was not functional so we used externally attached Ashling Opella-XD
probe. But now with production units everything works as expected and
so we anybody may enjoy readily avaialble built-in JTAG probe so
we specify Digilent oprion on MDB's command line example.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>