net/mlx5: Add the log_min_mkey_entity_size capability

Add the capability that will allow the driver to determine the minimal
MTT page size to be able to map the smallest possible pages in XSK. The
older firmwares that don't have this capability default to 12 (i.e.
4096-byte pages).

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>