dt-bindings: hwmon: pwm-fan: Document after shutdown fan settings

Document fan-shutdown-percent property, used to describe fan RPM in percent
set during shutdown. This is used to keep the fan running at fixed RPM after
the kernel shut down, which is useful on hardware that does keep heating
itself even after the kernel did shut down, for example from some sort of
management core.

Signed-off-by: Marek Vasut <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>

Documentation/devicetree/bindings/hwmon/pwm-fan.yaml

···
       it must be self resetting edge interrupts.
     maxItems: 1
 
+  fan-shutdown-percent:
+    description:
+      Fan RPM in percent set during shutdown. This is used to keep the fan
+      running at fixed RPM after the kernel shut down, which is useful on
+      hardware that does keep heating itself even after the kernel did shut
+      down, for example from some sort of management core.
+    minimum: 0
+    maximum: 100
+
   fan-stop-to-start-percent:
     description:
       Minimum fan RPM in percent to start when stopped.