libata-link: add PMP links

Add link->pmp, ap->nr_pmp_links, ap->pmp_link[], and implement/update
link helpers.

printk helpers are updated such that port and link are identifed as
'ataP:' if no PMP is attached, while device is identified as
'ataP.DD:'.  If PMP is attached, they become 'ataP:', 'ataP.LL:' and
'ataP.LL' - ie. link and device are identified their PMP number.

If PPM is attached (ap->nr_pmp_links != 0), ata_for_each_link()
iterates over PMP links, while __ata_for_each_link() iterates over the
host link + PMP links.  If PMP is not attached (ap->nr_pmp_links ==
0), both iterate over only the host link.

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