dt-bindings: PCI: cadence: Add DT bindings for Cadence PCIe host controller

This patch adds documentation for the DT bindings of the Cadence PCIe
controller when configured in host (Root Complex) mode.

Signed-off-by: Scott Telford <stelford@cadence.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@free-electrons.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>