Add experimental support for SATA interface power management.

Feature is controlled by hint.ata.X.pm_level tunable:
 0 - PM disabled, old behaviour, default.
 1 - device is allowed to initiate PM state change, host is passive.
 2 - host initiates PARTIAL state transition every time port is idle.
 3 - host initiates SLUMBER state transition every time port is idle.

PARTIAL state has up to 100us (50us for me) wakeup latency, but for my
ICH8M saves 0.5W of power per drive. SLUMBER state has up to 10ms (3.5ms
for me) wakeup latency, but saves 0.8W of power.

Modes 2 and 3 are implemented only for AHCI driver now.

Interface power management is incompatible with device presence detection
(host receives no signal from drive, so unable to monitor it), so later is
disabled when PM is used.

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