ata: ahci_platform: allow disabling of hotplug to save power

A number of resources remain powered to support hotplug. On platforms
I've worked with, allowing the ahci_platform to suspend saves about
150mW. This patch enables rpm and allows the device to be auto-suspended
through sysfs.

Signed-off-by: Samuel Morris <samorris@lexmark.com>
Signed-off-by: Tejun Heo <tj@kernel.org>