ahci_imx: Put #ifdef CONFIG_PM_SLEEP around suspend / resume functions

This fixes the following warnings when CONFIG_PM_SLEEP is not set:

drivers/ata/ahci_imx.c:284:12: warning: ‘imx_ahci_suspend’ defined but not used [-Wunused-function]
drivers/ata/ahci_imx.c:299:12: warning: ‘imx_ahci_resume’ defined but not used [-Wunused-function]

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