ath(4): begin fleshing out a "reset type" extension to force cold/warn resets.

Right now the only way to force a cold reset is:

• The HAL itself detects it's needed, or
• The sysctl, setting all resets to be cold.

Trouble is, cold resets take quite a bit longer than warm resets.

However, there are situations where a cold reset would be nice.
Specifically, after a stuck beacon, BB/MAC hang, stuck calibration results,
etc.

The vendor HAL has a separate method to set the reset reason (which is
how HAL_RESET_BBPANIC gets set) which informs the HAL during the reset path
why it occured. This is almost but not quite the same; I may eventually
unify both approaches in the future.

This commit just extends HAL_RESET_TYPE to include both status (eg BBPANIC)
and type (eg do COLD.) None of the HAL code uses it yet though; that'll
come later.

It also is a big no-op in each HAL - I need to go teach each of the HALs
about cold/warm reset through this path.