dt-bindings: pci: xilinx-nwl: Document optional clock property

Clock property hasn't been documented in binding document but it is used
for quite a long time where clock was specified by commit 9c8a47b484ed
("arm64: dts: xilinx: Add the clock nodes for zynqmp").

Link: https://lore.kernel.org/r/67aa[e-mail]
Signed-off-by: Michal Simek <[e-mail]>
Signed-off-by: Lorenzo Pieralisi <[e-mail]>
Acked-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/pci/xilinx-nwl-pcie.txt

···
 
 Optional properties:
 - dma-coherent: present if DMA operations are coherent
+- clocks: Input clock specifier. Refer to common clock bindings
 
 Example:
 ++++++++