sys/power: Sleep type reporting by PM backends

Allow PM backends to report supported sleep types when registering
through power_pm_register. Expose this information through
kern.power.supported_stype sysctl, and set defaults for
power_standby/suspend/hibernate_stype based on this.

Implement this in ACPI PM backend.

Reviewed by: cy
Approved by: cy
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52727
Event: EuroBSDcon 2025 Devsummit