IB/mlx5: Add support for DEVX general command

Add support to run general firmware command via the DEVX interface.

A command that works on some object (e.g. CQ, WQ, etc.) will be added
in next patches while maintaining the required object lock.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>