Migrate the LNA mixing diversity machinery from the AR9285 HAL to the driver.

The AR9485 chip and AR933x SoC both implement LNA diversity.
There are a few extra things that need to happen before this can be
flipped on for those chips (mostly to do with setting up the different
bias values and LNA1/LNA2 RSSI differences) but the first stage is
putting this code into the driver layer so it can be reused.

This has the added benefit of making it easier to expose configuration
options and diagnostic information via the ioctl API.  That's not yet
being done but it sure would be nice to do so.

Tested:

* AR9285, with LNA diversity enabled
* AR9285, with LNA diversity disabled in EEPROM
Set the antenna "config group" field.

The reference HAL pushes a config group parameter to the driver layer
to inform it which particular chip behaviour to implement.

This particular value tags it as an AR9285.