dt-bindings: PCI: xilinx-nwl: Modify ECAM size in the DT example

Update ECAM size in the devicetree example to allow for the discovery of
up to 256 buses.

[kwilczynski: commit log]
Link: https://lore.kernel.org/linux-pci/20231016051102.1180432-3-thippeswamy.havalige@amd.com


Signed-off-by: Thippeswamy Havalige <thippeswamy.havalige@amd.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>