MAINTAINERS: add gxp fan controller and documents

Add the gxp-fan-ctrl.c and gxp-fan-ctrl.rst in hwmon
driver/documentation.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Reviewed-by: Guenter Roek <linux@roek-us.net>
Link: https://lore.kernel.org/r/20230103203654.59322-6-nick.hawkins@hpe.com


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