vdpa/mlx5: Expose descriptor group mkey hw capability

Necessary for improved live migration flow. Actual support will be added
in a downstream patch.

Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Link: https://lore.kernel.org/r/20230928164550.980832-3-dtatulea@nvidia.com


Signed-off-by: Leon Romanovsky <leon@kernel.org>