staging: mt7621-dts: add missing device_type in pcie root ports

According to the YAML schema 'pci-bus.yaml' the 'device_type'
property is mandatory for all pcie root ports. Hence add it.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210506170742.28196-3-sergio.paracuellos@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>