dt-bindings: pwm: argon40,fan-hat: Document Argon40 Fan HAT

Document trivial PWM on Argon40 Fan HAT, which is a RaspberryPi
blower fan hat which can be controlled over I2C.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Link: https://lore.kernel.org/r/20250629220757.936212-2-marek.vasut+renesas@mailbox.org


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>