dt-bindings: PCI: dra7xx: Add properties to enable x2 lane in dra7

Add syscon properties required for configuring PCIe in x2 lane mode.

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