dt-bindings: PCI: dwc: rockchip: Document optional PCIe reference clock input

On some boards the 100MHz PCIe reference clock to both controller and
devices is controllable. Add that clock to the list of clocks.

The clock is optional, so the minItems stays the same.

Link: https://lore.kernel.org/linux-pci/20231206145041.667900-1-heiko@sntech.de
Signed-off-by: Heiko Stuebner <heiko.stuebner@cherry.de>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>