sata/mvebu: use #ifdef around suspend/resume code

The newly added suspend/resume implementation for ahci_mvebu causes
a link error when CONFIG_PM_SLEEP is disabled:

ERROR: "ahci_platform_suspend_host" [drivers/ata/ahci_mvebu.ko] undefined!
ERROR: "ahci_platform_resume_host" [drivers/ata/ahci_mvebu.ko] undefined!

This adds the same #ifdef here that exists in the ahci_platform driver
which defines the above functions.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: d6ecf158 ("ata: ahci_mvebu: add suspend/resume support")
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Tejun Heo <tj@kernel.org>