mem-ruby: fix Sequencer latency reporting
mem-ruby: fix Sequencer latency reporting

Most protocols won't set initialRequestTime, forwardRequestTime, and
firstResponseTime, thus leading to calculated latency being always 0.

Change-Id: Id2c73314a964bf6efb262d40b3df515373023b84
Signed-off-by: Tiago Mück <02f879f33eea3aaf6781160757a59709a6f01bed@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41114
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5