dt-bindings: hwmon: pwm-fan: Support multiple fan tachometer inputs

Document and give an example of how to define multiple fan tachometer
inputs for the pwm-fan driver.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200920180943.352526-2-pbarker@konsulko.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>