systemc: Export the sc_core::sc_time class to python.

This class isn't incredibly useful in python, but it's needed to call
some other functions which are more useful.

Change-Id: I5c23cca0b50f0455423399db8b009bdf86a6ec41
Reviewed-on: https://gem5-review.googlesource.com/c/16502
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>