power: add environment capability probing

power: add environment capability probing

Currently, there is no way to know if the power management env is
supported without trying to initialize it. The init API also does
not distinguish between failure due to some error and failure due to
power management not being available on the platform in the first
place.

Thus, add an API that provides capability of probing support for a
specific power management API.

Suggested-by: Jerin Jacob <jerinj@marvell.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>