mem: Use a const T & in write<> to avoid an unnecessary copy.

mem: Use a const T & in write<> to avoid an unnecessary copy.

If the type T is complex/large, the it makes sense to access it in place
and not copy it and then not modify it.

Change-Id: Idd24be4fbba636375637ff72b1ba5ee32eb76215
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18573
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>