systemc: Rename tlm_core header files to have a .hh extentension.

This makes those files more consistent with other headers in gem5. The
top level headers (tlm and tlm.h) and the tlm_utils headers were left
alone since the user might reasonably expect those to have a well known
name. The tlm_core headers by comparison are an implementation detail,
and users shouldn't include them directly by name.

Change-Id: Ibc82e8159e47717c79d76f50ed96fdd619294a06
Reviewed-on: https://gem5-review.googlesource.com/c/15067
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>