ahci: implement PMP support

Implement AHCI PMP support.  ahci only supports command based
switching.  Also, for some reason, NCQ over PMP doesn't work now.
Other than that, everything works.

Tested on ICH9R, JMB360/363 + SIMG3726, 4726 and 5744.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Forrest Zhao <forrest.zhao@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>