dt-bindings: thermal: add support for Broadcom's Northstar thermal

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>