Fix PM recognition on recent Marvell boards

PM status is only supported on Kirkwood and Disvovery.
Cleanup the code to properly report its state on
other platforms.

Submitted by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D10718