website: Add ARM power and DVFS documentation, update FS ARM.

This is a squash of 4 commits. This commit:
- Adds a section to Learning gem5 part1_6 on how to set up and run an
  ARM FS simulation.
- Adds documentation and example code for adding power modelling to an
  ARM FS simulation, describing the different components required.
  This is placed in Learning gem5 part2_7.
- Adds documentation and example code for adding DVFS support to an ARM
  FS simulation, describing and explaining Clock Domains and Voltage
  Domains in the process.
  This is placed in Learning gem5 part2_8.
- Updates some broken or deprecated links and references in the
  documentation on building a Linux kernel for FS ARM simulations.

The new files relate to:

Jira: https://gem5.atlassian.net/browse/GEM5-801
Jira: https://gem5.atlassian.net/browse/GEM5-783

(and somewhat also related:
 Jira: https://gem5.atlassian.net/browse/GEM5-463
 Jira: https://gem5.atlassian.net/browse/GEM5-769
)

Change-Id: Ia0375c94b2ae6441cfd3e6f114625934f1df5b37
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/36555
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
6 files changed