dt-bindings: PCI: Add dt-binding to configure PCIe mode

Add "ti,syscon-pcie-mode" dt-binding to hold phandle to the syscon
register that should be used to configure PCIe in RC mode or EP mode.

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>