ata: sata_mv: Fix probe failures with optional phys

Make use of devm_phy_optional_get() in order to fix probe failures on
Armada 370, XP and others, when there is no phy driver available.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>