mem: Add a readString method to the PortProxy which takes a char *.

mem: Add a readString method to the PortProxy which takes a char *.

This version takes a char * instead of an std::string &, and a maximum
length to fill in like strncpy. This is intended to be a replacement
for the CopyStringOut function.

Change-Id: Ib661924a3fa7e05761d572ffecbe2c0cc8659d48
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18574
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>