ata: ahci: add support for DaVinci DM816 SATA controller

This SATA controller is quite similar to the one present on the DA850
SoC, but the PHY configuration is different and it supports two HBA
ports.

The IP suffers from the same PMP issue the DA850 does - if we enable
PMP but don't use it - softreset fails. Appropriate workaround was
implemented in this driver as well.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Tejun Heo <tj@kernel.org>