ixl: implement support for dumping NIC registers

[pkubaj]

Summary

To enable faster debug of customer issues Intel wants an end user to
gather information about PHY configuration and status to debug link and
connection issues in the field. For this reason the end user should
have possibility of obtaining PHYs registers values at runtime during
debugging.

[pkubaj]

@erj
Can you either commit this patch or accept as a source committer so that I can commit?

[erj]

@pkubaj I can just accept it -- it looks good to me.

I do wish that they did this feature for X722, though. :p