Sorry. I think I didn't discribe exactly what I want to do. Because patching the kernel which would allow to set it within DTS is not a good idea. Despite I think it would not be allowed.
What I would do is to just put a script into /etc/init.d respectively /etc/rc.d to set this parameter on boot time. Is this acceptable or not?
I mean I could it just put into rc.local. But that is not the best idea (imo).
EDIT:
I would do sth. like that (if this is allowed to be integrated in OpenWrt):
#!/bin/sh /etc/rc.common
START=99
boot() {
# get board name
case $(board_name) in
zyxel_nbg7815)
# get correct thermal zone defined in DTS
aqr_thermal=$(grep -Ril "aqr-thermal" /sys/class/thermal/thermal_zone*/type | cut -d'/' -f 5)
# setting thermal governor from step_wise to bang_bang
echo "bang_bang" > /sys/class/thermal/${aqr_thermal}/policy
;;
esac
}
While I'm not sure if it would make sense to set this for all zones.
EDIT:
Does the required kernel parameter have a chance to be accepted? Because kernel config is for all targets. At least I didn't found a file which would pharse kernel settings for a specific device. 
Let's say it in other way: The MAC's in ART are clearly not uniqe. So maybe OpenWrt is deciding within a script which I'm not aware of to alter them because of this.