device_type = "pci" should be used for the SoC level PCI controller and
it is already present at imx6qdl.dtsi.

Remove it from the subnode in order to fix the following build
warnings with W=1:

arch/arm/boot/dts/imx6q-zii-rdu2.dtb: Warning (pci_bridge): Node /soc/pcie@1ffc000/pci@0 missing ranges for PCI bridge (or not a bridge)
arch/arm/boot/dts/imx6q-zii-rdu2.dtb: Warning (pci_bridge): Node /soc/pcie@1ffc000/pci@0 missing bus-range for PCI bridge
arch/arm/boot/dts/imx6q-zii-rdu2.dtb: Warning (unit_address_format): Failed prerequisite 'pci_bridge'
arch/arm/boot/dts/imx6q-zii-rdu2.dtb: Warning (pci_device_reg): Failed prerequisite 'pci_bridge'
arch/arm/boot/dts/imx6q-zii-rdu2.dtb: Warning (pci_device_bus_num): Failed prerequisite 'pci_bridge'
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>