ata: ahci: mvebu: request PHY suspend/resume for Armada 3700

A feature has been added in the libahci driver: the possibility to set
a new flag in hpriv->flags to let the core handle PHY suspend/resume
automatically. Make use of this feature to make suspend to RAM work
with SATA drives on A3700.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>