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 9c8a47b4


("arm64: dts: xilinx: Add the clock nodes for zynqmp").

Link: https://lore.kernel.org/r/67aa2c189337181bb2d7721fb616db5640587d2a.1624618100.git.michal.simek@xilinx.com
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>