libata: Add the AHCI_HFLAG_YES_ALPM flag

Some hardware is capable of supporting Aggresive Link Power Management
even though it is not indicated by the Host Capability register.

This commit adds the AHCI_HFLAG_YES_ALPM flag to the AHCI library to
allow indication of this quirk when the Host Capability register is
Read Only and therefore cannot be changed.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>