ahci_xgene: Implement the workaround to support PMP enumeration and discovery.

Due to H/W errata, the controller is unable to save the PMP
field fetched from command header before sending the H2D FIS.
When the device returns the PMP port field in the D2H FIS, there is
a mismatch and results in command completion failure. The workaround
is to write the pmp value to PxFBS.DEV field before issuing any command
to PMP.

Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>