[dv/edn] Increase edn genbit max size
[dv/edn] Increase edn genbit max size

This PR increases genbits max size to increase toggle and functional
coverage.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@opentitan.org>

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