This commit documents binding for thermal used in Northstar family SoCs.

There isn't any known Northstar device with active cooling system so DT
example has empty cooling-maps node. There is also no support for CPU
frequency throttling so I put only a critical trip in the example.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>