PCI: artpec6: Configure FTS with dwc helper function

Use DesignWare helper functions to configure Fast Training
Sequence. Drop the respective code in the driver.

Signed-off-by: Dilip Kota <eswara.kota@linux.intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>