tests/pm_pc8: try harder to reach PC8+ residencies

tests/pm_pc8: try harder to reach PC8+ residencies

If we want to actually reach PC8+ states, we need to properly
configure all the devices on the system to allow this. This function
will try to setup the things we know we need, but won't scream in case
anything fails: we don't know which devices are present on your
machine, so we can't really expect anything, just try to help with the
more common problems.

Another reason for this commit is that I got tired of having to
readjust the runtime PM policies every time I reboot my machine.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>