Document the AR9285/AR9485 LNA configuration information that's stored in the ctl/ext RSSI field for chain 2.
Document the AR9285/AR9485 LNA configuration information that's
stored in the ctl/ext RSSI field for chain 2.

Tested:

* AR9285, STA

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase