systemc: Add some deprecated sc_time factory methods.

Change-Id: I0d9a7040a48b9f0d0079e9daecaf44ea78c186de
Reviewed-on: https://gem5-review.googlesource.com/11273
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>