libata: Export AHCI capabilities

AHCI exports various capability bits that may be of interest to userspace
such as whether the BIOS claims a port is hotpluggable or eSATA. Providing
these via sysfs along with the version of the AHCI spec implemented by
the host allows userspace to make policy decisions for things like ALPM.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>