power-mgmt: add per-domain control loop

Currently there is support for one temperature sensor
and a number of actors are controlled by one single loop.
There are however examples of applications where it could be
necessary to have multiple independent thermal domains.
This patch adds the option to add multiple thermal management
controllers using different temperature sensors, in order to
have multiple loops working at the same time.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I92182e9ddd21d8d35ec1e47a7286eea482288508