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: NTejun Heo <htejun@gmail.com>
Cc: Forrest Zhao <forrest.zhao@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>