ahci_platform: enable hotplug unbinding

platform_driver_probe() should be used for registering this driver only
if we want to

    "...remove its run-once probe() infrastructure from memory after the
    driver has bound to the device."

However, we may want to leave the probe infrastructure in place in order
to support binding/unbinding a device dynamically. This is useful, for
instance, as a power management mechanism, where a device can be totally
powered down when unbound (whereas with runtime power management,
powering down the SATA core would incur unacceptable loss of
functionality).

Thus, convert this driver to use platform_driver_register().

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