colibri_t20: integrate thermal throttling

As our hardware only allows triggering an interrupt on over-temperature
shutdown we first use it to catch entering throttle limit and only then
set it up to catch an actual over-temperature shutdown.
While throttling we setup a workqueue to catch leaving it again.

We support the same sysfs interface as on NVIDIA Ventana:

root@colibri-t20:~# cat /sys/kernel/debug/thermal/shutdown
115000
root@colibri-t20:~# cat /sys/kernel/debug/thermal/throttle
90000

As follows some output of before and during a throttling run:

root@colibri-t20:~# tegrastats
RAM 159/345MB (lfb 36x4MB) Carveout 11/128MB (lfb 116MB) GART 0/32MB
(lfb 32MB) IRAM 4/255kB(lfb 248kB) cpu [51%,23%]@216 EMC 666 AVP 72 VDE
240 EDP limit -1 Temperatures CPU 48.7 Board 43.2

root@colibri-t20:~# cat /sys/devices/system/cpu/cpu0/cpufreq/throttle
0

root@colibri-t20:~# cat /sys/bus/i2c/devices/4-004c/temp2_os
90000

root@colibri-t20:~# cat /sys/kernel/debug/thermal/shutdown
115000
root@colibri-t20:~# cat /sys/kernel/debug/thermal/throttle
90000

root@colibri-t20:~# tegrastats
RAM 66/345MB (lfb 5x32kB) Carveout 75/128MB (lfb 53MB) GART 8/32MB
(lfb 24MB) IRAM 20/255kB(lfb 232kB) cpu [85%,83%]@912 EMC 666 AVP 216
VDE 240 EDP limit -1 Temperatures CPU 90.1 Board 77.9

root@colibri-t20:~# cat /sys/devices/system/cpu/cpu0/cpufreq/throttle
1

root@colibri-t20:~# cat /sys/bus/i2c/devices/4-004c/temp2_os
115000

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