net/mlx5: EQ, Generic EQ

Add mlx5_eq_{create/destroy}_generic APIs and EQE access methods, for
mlx5 core consumers generic EQs.

This API will be used in downstream patch to move page fault (RDMA ODP)
EQ logic into mlx5_ib rdma driver, hence it will use a generic EQ.

Current mlx5 EQ allocation scheme:
On load mlx5 allocates 4 (for async) + #cores (for data completions)
MSIX vectors, mlx5 core will assign 3 MSIX vectors for internal async
EQs and will use all of the #cores MSIX vectors for completion EQs,
(One vector is going to be reserved for a generic EQ).

After this patch an external user (e.g mlx5_ib) of mlx5_core
can use this new API to create new generic EQs with the reserved msix
vector index for that eq.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>