tlm: Don't set SystemC time resolution

tlm: Don't set SystemC time resolution

Some simulators already set the time resolution of SystemC. By coupling
gem5 by means of SystemC with an other tool this can lead to problems:
When the resolution is set twice the simulation will not work.
Therefore, the line is changed to an assertion that checks if the time
resolution of the SystemC simulation is set to gem5's value of 1ps.

Change-Id: I8aafab9dd593eb4697a3c7f69ce181fd9cdd05c4
Reviewed-on: https://gem5-review.googlesource.com/4520
Reviewed-by: Paul Rosenfeld <prosenfeld@micron.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>