net/mlx4_core: Maintain a persistent memory for mlx4 device

Maintain a persistent memory that should survive reset flow/PCI error.
This comes as a preparation for coming series to support above flows.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>