dt-bindings: ata: ahci: Add platform capability properties

In case if the platform doesn't have BIOS or a comprehensive firmware
installed then the HBA capability flags will be left uninitialized. As a
good alternative we suggest to define the DT-properties with the AHCI
platform capabilities describing all the HW-init flags of the
corresponding capability register. Luckily there aren't too many of them.
SSS - Staggered Spin-up support and MPS - Mechanical Presence Switch
support determine the corresponding feature availability for the whole HBA
by means of the "hba-cap" property. Each port can have the "hba-port-cap"
property initialized indicating that the port supports some of the next
functionalities: HPCP - HotPlug capable port, MPSP - Mechanical Presence
Switch attached to a port, CPD - Cold Plug detection, ESP - External SATA
Port (eSATA), FBSCP - FIS-based switching capable port.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>