soc/hps_soc.py: Add parallel flag
soc/hps_soc.py: Add parallel flag

Adds command line flag to run multiple instances of nextpnr in parallel.

Signed-off-by: Alan Green <97e77871b5fcc4941bebceb4338c97c1fd149f8c@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground