ata: add AMD Seattle platform driver

AMD Seattle SATA controller mostly conforms to AHCI interface with some
special register to control SGPIO interface. In the case of an AHCI
controller, the SGPIO feature is ideally implemented using the
"Enclosure Management" register of the AHCI controller, but those
registeres are not implemented in the Seattle SoC. Instead SoC
(Rev B0 onwards) provides a 32-bit SGPIO control register which should
be programmed to control the activity, locate and fault LEDs.

The driver is based on ahci_platform driver.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
CC: tj@kernel.org
CC: linux-ide@vger.kernel.org
Signed-off-by: Tejun Heo <tj@kernel.org>