PWCache latency not added into the mmu.total_walk_latency

[jonghyunL]

While PWCache access_latency and miss_latency is defined by the configuration,
the corresponding cycles are not added into the total_walk_latency in common/core/memory_subsystem/parametric_dram_directory_msi/mmu_designs/(mmu.cc, mmu_*.cc)::filterPTWResult()

I think the fix should be fairly straightforward, by adding getLatency() function in pwc.h like below and calling it in filterPTWResult() function to add the latencies.

This is very small number of cycles that would be added into the total_walk_latency.