sim: Use the Temperature type in power/thermal models

sim: Use the Temperature type in power/thermal models

The thermal models currently work on temperatures in Celsius stored in
plain doubles. Switch to using Temperature instead and internal
processing in Kelvin. There should be no impact on the result since
all thermal processes work on temperature deltas.

Change-Id: I22d0261ae102f30d86051f24a2d88b067b321c91
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39455
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>