Documentation/ABI: Add document for Mellanox PMC driver

Documentation/ABI: Add document for Mellanox PMC driver

Document the sysfs interface for programming and monitoring the
performance counters in various HW blocks of Mellanox BlueField-1,
BlueField-2 and BlueField-3.

Signed-off-by: Shravan Kumar Ramani <shravankr@nvidia.com>
Reviewed-by: David Thompson <davthompson@nvidia.com>
Link: https://lore.kernel.org/r/9a9e69bd99cad3ad8d1847a6e4e10aff80c6df50.1736413033.git.shravankr@nvidia.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>