ahci: sis can't do PMP

From: Piter PUNK <piterpunk@slackware.com>

SiS AHCIs say they can do PMP but can't and fail detection if SRST w/
pmp==15 is used.  Turn off PMP support.

tj: added patch description, adapted patch to #upstream-fixes and
    renamed board_ahci_sis to board_ahci_nopmp.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>