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

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 <marek.vasut+renesas@mailbox.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250904202157.170600-1-marek.vasut+renesas@mailbox.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>