ata: ahci_platform: Enable runtime power management

Provide runtime power management callbacks for AHCI device.
The runtime power management will be disabled by default
due to regressions in hot plug functionality, but userspace
can enable it by echo'ing into following sysfs entry:

echo auto > /sys/devices/platform/ahci.X/power/control

Change-Id: I8421fc1369b45eb76dd0773a512984fd4fee99b4
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>