dt-bindings: thermal: give OS some leeway in absence of critical-action

An operating system may allow its user to configure the action to be
undertaken on critical overtemperature events.

However, the bindings currently mandate an absence of the critical-action
property to be equal to critical-action = "shutdown", which would mean any
differing user configuration would violate the bindings.

Resolve this by documenting the absence of the property to mean that the
OS gets to decide.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ahmad Fatoum <[e-mail]>
Acked-by: Rob Herring (Arm) <[e-mail]>
Cc: Benson Leung <[e-mail]>
Cc: Daniel Lezcano <[e-mail]>
Cc: Fabio Estevam <[e-mail]>
Cc: Guenter Roeck <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Liam Girdwood <[e-mail]>
Cc: Lukasz Luba <[e-mail]>
Cc: Mark Brown <[e-mail]>
Cc: Matteo Croce <[e-mail]>
Cc: Matti Vaittinen <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: Rui Zhang <[e-mail]>
Cc: Sascha Hauer <[e-mail]>
Cc: "Serge E. Hallyn" <[e-mail]>
Cc: Tzung-Bi Shih <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/devicetree/bindings/thermal/thermal-zones.yaml

···
         $ref: /schemas/types.yaml#/definitions/string
         description: |
           The action the OS should perform after the critical temperature is reached.
-          By default the system will shutdown as a safe action to prevent damage
-          to the hardware, if the property is not set.
-          The shutdown action should be always the default and preferred one.
+          If the property is not set, it is up to the system to select the correct
+          action. The recommended and preferred default is shutdown.
           Choose 'reboot' with care, as the hardware may be in thermal stress,
           thus leading to infinite reboots that may cause damage to the hardware.
           Make sure the firmware/bootloader will act as the last resort and take