systemc: Remove redundant tlm_ prefixes from file names.

We already know those files belong to tlm because of the directory
they're in. Removing the prefix makes the paths of the headers less
enormously long.

Change-Id: I869e58fae904162f353bb31f4c0919fba08dffa6
Reviewed-on: https://gem5-review.googlesource.com/c/15059
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>