enic: Add support for firmware management device

This patch adds support for firmware management device in enic driver.
A management device is a virtual PCIe device that firmware can present
to the host as its management endpoint. It provides the interface between
the host and adapter firmware for all management operations.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: Roopa Prabhu <roprabhu@cisco.com>
Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>