mb/asrock/h77pro4-m: Make onboard NIC a child device below PCIe port 6

The Realtek RTL8111E NIC is currently not defined as a child device,
resulting in the on_board flag not being set to 1. This means that
Linux / udev will call the device enp4s0 rather than eno0, as is
appropriate for on-board ethernet devices.

This patch defines the NIC as a child device of PCIe port 6, so that
it's properly defined as an on-board device.

Change-Id: I2e1b65e4d27852297a739e332c52c15a8c81b858
Signed-off-by: Kevin Keijzer <kevin@quietlife.nl>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74090
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/asrock/h77pro4-m/devicetree.cb

@@ -47,6 +47,7 @@ chip northbridge/intel/sandybridge
 			end
 			device ref pcie_rp6 on # PCIe Port #6 - RTL8111E GbE
 				subsystemid 0x1849 0x1e1a
+				device pci 00.0 on end # PCI 10ec:8168
 			end
 			device ref pcie_rp7 on # PCIe Port #7 - slot "PCIE3", 1 lane
 				subsystemid 0x1849 0x1e16