libahci: Allow drivers to override stop_engine

Marvell armada37xx, armada7k and armada8k share the same
AHCI sata controller IP, and currently there is an issue
(Errata Ref#226)that the SATA can not be detected via SATA
Port-MultiPlayer(PMP). After debugging, the reason is
found that the value of Port-x FIS-based Switching Control
(PxFBS@0x40) became wrong.
According to design, the bits[11:8, 0] of register PxFBS
are cleared when Port Command and Status (0x18) bit[0]
changes its value from 1 to 0, i.e. falling edge of Port
Command and Status bit[0] sends PULSE that resets PxFBS
bits[11:8; 0].
So it needs save the port PxFBS register before PxCMD
ST write and restore the port PxFBS register afterwards
in ahci_stop_engine().

This commit allows drivers to override ahci_stop_engine
behavior for use by the Marvell AHCI driver(and potentially
other drivers in the future).

Signed-off-by: Evan Wang <xswang@marvell.com>
Cc: Ofer Heifetz <oferh@marvell.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Tejun Heo <tj@kernel.org>