dt-bindings: PCI: cadence: Add DT bindings for optional PHYs

Update DT documentation to include optional PHYs for cadence PCIe
host and endpoint controllers.

Signed-off-by: Alan Douglas <adouglas@cadence.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>