ena: Add configuration notifications interface support

This commit is part of the effort of notifying the user of non-optimal
or performance impacting practices.
A new interface is serving as a communication channel
between the device and the driver. One of the goals of this channel is
to create a new mechanism of notifying the driver and user in case of
sub-optimal configuration using a bitmap.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.