dt-bindings: PCI: MediaTek: fix dtc warnings

dtc recently added PCI bus checks. Fix these warnings:

Warning (pci_bridge): Node /pcie@1a140000/pcie@0,0 missing bus-range for PCI bridge
Warning (pci_bridge): Node /pcie@1a140000/pcie@1,0 missing bus-range for PCI bridge
Warning (pci_bridge): Node /pcie@1a140000/pcie@2,0 missing bus-range for PCI bridge
Warning (unit_address_format): Failed prerequisite 'pci_bridge'
Warning (pci_device_reg): Failed prerequisite 'pci_bridge'
Warning (pci_device_bus_num): Failed prerequisite 'pci_bridge'

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>