PCI: keystone: Cleanup PHY handling

Cleanup PHY handling by using devm_phy_optional_get() to get PHYs if
the PHYs are optional, creating a device link between the PHY device
and the controller device and disable PHY on error cases here.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>