ata: add platform driver for Calxeda AHCI controller

Calxeda highbank SATA phy has intermittent problems bringing up a link
with Gen3 drives. Retrying the phy hard reset can work-around this issue,
but each reset also disables spread spectrum support. The reset function
also needs to reprogram the phy to enable spread spectrum support.

Create a new driver based on ahci_platform to support the Calxeda Highbank
SATA controller.

Signed-off-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>