net/mlx5e: Expose memory key creation (mkey) function

Expose mlx5e_create_mkey function, for future patches in the
macsec series to use.
The above function creates a memory key which describes a
region in memory that can be later used by both HW and SW.
The counterpart destroy functionality is already exposed.

Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Emeel Hakim <ehakim@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>