ahci: Add runtime PM support for the host controller

This patch adds runtime PM support for the AHCI host controller driver so
that the host controller is powered down when all SATA ports are runtime
suspended. Powering down the AHCI host controller can reduce power
consumption and possibly allow the CPU to enter lower power idle states
(S0ix) during runtime.

Runtime PM is blocked by default and needs to be unblocked from userspace
as needed (via power/* sysfs nodes).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>