[fpv] update jg tcl script for 2020.12 new version
[fpv] update jg tcl script for 2020.12 new version

The JapserGold 2020.12 version includes a feature to randomize input
data at both pos/nedgedge clocks if -both-edge switch is enabled. But
our design's data should only be randomized at one clock edge.
This PR fixes it by adding a `-rate` to constraint only randomize at
posedge clock.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan