dt-bindings: PCI: keystone: Add bindings to get device control module

Add bindings to get device control module which has the device id and
vendor id to be configured in the keystone PCIe controller.

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>