linux-phytec-ti

dt-bindings: PCI: designware-pcie: Document "num-ob-windows" to be optional

Certain platforms like TI's K2G manages it's own outbound windows.
For such platforms "num-ob-windows" property is not required.
Document it to be optional here.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>