dt-bindings: PCI: cadence: Add PCIe RC/EP DT schema for Cadence PCIe

Add PCIe Host (RC) and Endpoint (EP) device tree schema for Cadence
PCIe core library. Platforms using Cadence PCIe core can include the
schemas added here in the platform specific schemas.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>