libata: convert drivers with combined SATA/PATA ports to new init model

Convert sata_via and sata_promise to new init model.  Both controllers
can have combined configuration (SATA + PATA) and used twisted
initialization method (modifying port in ->port_start) to overcome
probe_ent limitations.

This patch converts both drivers to new init model in which such
configuration is natively supported.

* promise: Combined pata port now uses separate port_info entry right
  after the sata counterpart entry.

* promise: Controller configuration is discerned using ap->flags.
  This simplifies init path and makes it look more like other LLDs.

* via: Both SATA and PATA ports in vt6421 are represented in their
  own port_info structure.

Tested on PDC20375 (SATA150 TX2plus) [105a:3375] and PDC40775 (SATA
300 TX2plus) [105a:3d73].  Couldn't test via cuz my c3 won't boot the
current kernel.

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