mem: Initialize pendingWrites stat of NVMStats
mem: Initialize pendingWrites stat of NVMStats

The initialization for this stat was mistakenly removed here
https://gem5-review.googlesource.com/c/public/gem5/+/39276

Signed-off-by: Hoa Nguyen <a0aa8449aacb303edcb25d6bbe69da9124f010fc@ucdavis.edu>
Change-Id: I27b6ddf6d6b925c9787acb07552a8d5acb081c22
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41073
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>
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