[dv/pwrmgr] Disable escalation ping fcov
[dv/pwrmgr] Disable escalation ping fcov

This PR disable the fcov collected by escalation agent regarding the
ping coverage. Because pwrmgr stimulus doesn't send out ping request.
This feature is covered in alert_handler testbench.

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

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