net/mlx5: Introduce inter-device communication mechanism

This introduces devcom, a generic mechanism for performing operations
on both physical functions of the same Connect-X card.

The first user of this API is merged eswitch, which will be introduced
in subsequent patches.

Signed-off-by: Aviv Heller <avivh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>