dt-bindings: PCI: mediatek,mt7621: Add missing child node reg

dt-bindings: PCI: mediatek,mt7621: Add missing child node reg

MT7621 PCI host bridge has children which are PCI root ports.  The
children have "reg" property, but do not explicitly define it.  Instead
they rely on pci-bus.yaml schema, but that one has "reg" without any
constraints.

Define the "reg" for the children, so the binding will be more specific
and later will allow dropping reference to deprecated pci-bus.yaml
schema.

Link: https://lore.kernel.org/linux-pci/20240413151617.35630-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>