Workaround fix for intermittent link issue with axgbe driver

[rajesh1.kumar_amd.com]

Summary

Intermittently, some link issues observed with axgbe driver. By default,
axgbe driver does a receiver reset after predefined number of retries for the
link to come up. But intermittently, this receiver reset doesn't suffice.
In that case, as a workaround we need to reset the phy completely.

This patch introduces a sysctl where which can be set to reset the phy
completely, rather than just doing receiver reset.

Test Plan

When trying with axgbe driver, if link doesn't comeup, the following sysctl
can be set as below.

Enable link workaround

dev.ax.X.link_workaround=1

[stephan.dewt_yahoo.co.uk]

Tested and confirmed to be working on the AMD SoC 10GbE NIC. Right now this is the only way to get a consistent link-up during boot and when hotplugging SFP+ modules. The internal receiver reset cycle might contain a hardware issue that prevents it from working properly.

[rajesh1.kumar_amd.com]

Thanks @stephan.dewt_yahoo.co.uk for testing and giving your comments.

@vmaffione and @manu, please let me know if you have any comments.

[rajesh1.kumar_amd.com]

Abandoning this revision and submitted another patch (D28266) which has this patch along with fixes for other known link issues with axgbe