MIPS: dts: Boston: Fix dtc 'pci_device_reg' warning

dtbs_check currently complains that:
arch/mips/boot/dts/img/boston.dts:128.19-178.5: Warning (pci_device_reg):
/pci@14000000/pci2_root@0,0,0: PCI unit address format error,
expected "0,0"
The unit-address format should be '<device>,<function>'.
Fix the unit-address accordingly.

Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: Genjian Zhang <zhanggenjian@kylinos.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>