systemc: Change how the default timescale is set.
systemc: Change how the default timescale is set.

The spec says that the default timescale should be 1 PS, but the
Accellera implementation uses the time resolution.

Change-Id: I7b307a33ef0856e9c19d81e401b15691275d4978
Reviewed-on: https://gem5-review.googlesource.com/c/12975
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu