Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Implement an API to fetch the default DFS parameters for the given chip.

The only chip this is currently implemented for is the AR5416 HAL family.
A follow-up commit will add AR5212 support.

PR: kern/170904

Break out the radar code into a separate source file.

This mirrors the internal HAL organisation and reduces the differences
between the HAL codebases slightly.

Obtained from: Atheros