[libata] AHCI: add hw link power management support

[libata] AHCI: add hw link power management support

This patch will set the correct bits to turn on Aggressive
Link Power Management (ALPM) for the ahci driver.  This
will cause the controller and disk to negotiate a lower
power state for the link when there is no activity (see
the AHCI 1.x spec for details).  This feature is mutually
exclusive with Hot Plug, so when ALPM is enabled, Hot Plug
is disabled.  ALPM will be enabled by default, but it is
settable via the scsi host syfs interface.  Possible
settings for this feature are:

Setting         Effect
----------------------------------------------------------
min_power       ALPM is enabled, and link set to enter
                lowest power state (SLUMBER) when idle
                Hot plug not allowed.

max_performance ALPM is disabled, Hot Plug is allowed

medium_power    ALPM is enabled, and link set to enter
                second lowest power state (PARTIAL) when
                idle.  Hot plug not allowed.

Signed-off-by:  Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>