mb/asrock/b75m-itx: Make NIC a child device below PCIe port 4

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 enp3s0 rather than eno0, as is
appropriate for on-board ethernet devices.

Additionally, the comment in devicetree.cb stating that PCIe port 6
is the ethernet controller is incorrect. It's actually port 4.

This patch moves the comment to the right port, and defines the NIC
as a child device of said port, so that it's properly defined as an
on-board device.

Link: https://mail.coreboot.org/hyperkitty/list/coreboot@coreboot.org/thread/TFWNW3Y7IWTFD4KIBVNQYW3DODJ6SSC2/

Change-Id: Ie1e3a757a6bd6c7dd1702ced177d13711978dcc4
Signed-off-by: Kevin Keijzer <kevin@quietlife.nl>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73516
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Fabian Groffen <grobian@gentoo.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/asrock/b75m-itx/devicetree.cb

@@ -50,11 +50,13 @@ chip northbridge/intel/sandybridge
 			end
 			device pci 1c.1 off end # PCIe Port #2
 			device pci 1c.2 off end # PCIe Port #3
-			device pci 1c.3 on  # PCIe Port #4
-				subsystemid 0x1849 0x1e16
+			device pci 1c.3 on  # PCIe Port #4, Realtek PCIe GbE Controller
+				device pci 00.0 on  # PCI 10ec:8168
+					subsystemid 0x1849 0x1e16
+				end
 			end
 			device pci 1c.4 off end # PCIe Port #5
-			device pci 1c.5 on  # PCIe Port #6, Realtek PCIe GbE Controller
+			device pci 1c.5 on  # PCIe Port #6
 				subsystemid 0x1849 0x1e1a
 			end
 			device pci 1c.6 off end # PCIe Port #7