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 <hoanguyen@ucdavis.edu>
Change-Id: I27b6ddf6d6b925c9787acb07552a8d5acb081c22
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41073
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>