hwmon: (pwm-fan) Implement after shutdown fan settings

hwmon: (pwm-fan) Implement after shutdown fan settings

Add 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. The current behavior of pwm-fan is to unconditionally
stop the fan on shutdown, which is not always the safe and correct thing
to do, so let the hardware description include the expected behavior.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20250904202157.170600-2-marek.vasut+renesas@mailbox.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>