systemc: Implement a little more of sc_time.
systemc: Implement a little more of sc_time.

This change implements a pair of constructors, and some conversion
functions.

Change-Id: Ibd0e9ec5bcc24f4a17896313996b3112b5ef1934
Reviewed-on: https://gem5-review.googlesource.com/12208
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

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