dt-bindings: hwmon: Add hpe,gxp-fan-ctrl

Create documentation for the binding to support control of the fans on
HPE platforms.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230103203654.59322-3-nick.hawkins@hpe.com


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