net/mlx5: devcom, Add component size getter

Add a getter for the number of participants in a devcom
component (those who share the same component id and key).

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>