sim, power: Temperature used for power calculations

sim, power: Temperature used for power calculations

The temperature used for the power calculations was fixed at 0
degrees, unless a thermal model was setup.  This commit allows
the user to set the temperature that needs to be used by the
power calculation during gem5 configuration.  This value will be
overwritten if there are thermal models present.

Change-Id: I7ca8fa6766bdcba9d362c12fc75d1e1f74385f35
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8602
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>