MAINTAINERS: Add documentation reference for Mellanox platform

MAINTAINERS: Add documentation reference for Mellanox platform

Add reference for Documentation/ABI/stable/sysfs-driver-mlxreg-io
under "MELLANOX HARDWARE PLATFORM SUPPORT".

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502032336.lauIV68J-lkp@intel.com/
Link: https://lore.kernel.org/r/20250210203451.29712-1-vadimp@nvidia.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>