Bring over the AR9285 board update code from ath9k.
Bring over the AR9285 board update code from ath9k.

This does a few things in particular:

* Abstracts out the gain control settings into a separate function;
* Configure antenna diversity, LNA and antenna gain parameters;
* Configure ob/db entries - the later v4k EEPROM modal revisions have
  multiple OB/DB parameters which are used for some form of
  calibration. Although the radio does have defaults for each,
  the EEPROM can override them.

This resolves the AR2427 related issues I've been seeing and makes
it stable at all 11g rates for both TX and RX.

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