net/mlx5: Split irq_pool_affinity logic to new file

The downstream patches add more functionality to irq_pool_affinity.
Move the irq_pool_affinity logic to a new file in order to ease the
coding and maintenance of it.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>