systemc: Add m5.systemc and m5.tlm python modules.

These will be how systemc and tlm APIs which are not attached to
SimObjects will be exposed. This avoids having to artificially attach
them to wrapping SimObjects for instance, which is a bit awkward
and non-obvious.

The python code which attaches the systemc and tlm modules to the
m5 modules lives in src/python/m5/__init__.py, but the modules
themselves live in src/systemc/python to keep all the systemc code
grouped together. It might be a little confusing to have a small part
of the glue that adds those modules in a separate place (__init__.py),
but that is, as far as I can tell, unavoidable, and it's better in my
opinion to keep the systemc code grouped together than to put it
alongside the other python code and __init__.py.

Change-Id: Iecb218daec5e15772152b5ad22b51f43b86c3d4b
Reviewed-on: https://gem5-review.googlesource.com/c/16563
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>