ahci: platform support for suspend/resume

Add platform hooks for custom suspend() and resume() functions. The
generic suspend/resume code in drivers/ata/ahci_platform.c is adapted
from the PCI version in drivers/ata/ahci.c.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>