libata-pmp-prep: implement ATA_HORKAGE_SKIP_PM

Some pseudo devices fail PM commands unnecessarily aborting system
suspend.  Implement ATA_HORKAGE_SKIP_PM which makes libata skip PM
commands for these devices.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>