util: Make dot_writer ignore NULL simobjects.
util: Make dot_writer ignore NULL simobjects.

Also centralize the code which iterates over the parameters of a simnode's
children, and remove a copy/paste block of code in the DVFS block of code.

Change-Id: I01305fbff59346010780ee386ba97ad9106b1f5d
Reviewed-on: https://gem5-review.googlesource.com/4849
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

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