libata: Add a per-host flag to opt-in into parallel port probes

This patch adds a per host flag that allows drivers to opt in into
having its busses scanned in parallel.

Drivers that do not set this flag get their ports scanned in
the "original" sequence.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>