[fpv/csr_assert] Improve CSR assertions
[fpv/csr_assert] Improve CSR assertions

This PR has a few improvement regarding csr assertions:
1. Reduce exp_vals size as Issue 5583 recommanded.
2. Constrain a_source value to avoid FPV running for too long

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

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