dt-bindings: PCI: mediatek,mt7621-pcie: Add PCIe host topology ASCII graph

MediaTek MT7621 PCIe sub-system supports a single Root Complex (RC)
with 3 Root Ports. Add PCIe host topology ASCII graph to the binding
for completeness.

Suggested-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/linux-pci/20240522044321.3205160-1-sergio.paracuellos@gmail.com


Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>