ata: move library code from ahci_platform.c to libahci_platform.c

Move AHCI platform library code from ahci_platform.c to
libahci_platform.c and fix dependencies for ahci_st,
ahci_imx and ahci_sunxi drivers.

Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>