ata: libahci_platform: Export again ahci_platform_<en/dis>able_phys()

This reverts commit 6bb86fef


("libahci_platform: Staticize ahci_platform_<en/dis>able_phys()") we are
going to need ahci_platform_{enable,disable}_phys() in a subsequent
commit for ahci_brcm.c in order to properly control the PHY
initialization order.

Also make sure the function prototypes are declared in
include/linux/ahci_platform.h as a result.

Cc: stable@vger.kernel.org
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>