mb/asrock/b75pro3-m: Advertise RTL NIC as onboard ethernet device

Move the onboard Realtek NIC definition to a child device of
PCIe port 6.  This makes sure it is advertised as "onboard", such that
it appears as eno0 on systemd/udev-based systems.

This commit is very similar to
https://review.coreboot.org/c/coreboot/+/73516

Signed-off-by: Fabian Groffen <grobian@gentoo.org>
Change-Id: I0550ee9faddd65011ad914aef413a6d1b316c5ad
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73519
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

src/mainboard/asrock/b75pro3-m/devicetree.cb

@@ -74,6 +74,7 @@ chip northbridge/intel/sandybridge
 			end
 			device ref pcie_rp6 on # PCIe Port #6, Realtek PCIe GbE Controller
 				subsystemid 0x1849 0x1e1a
+				device pci 00.0 on end
 			end
 			device ref pcie_rp7 off # PCIe Port #7
 			end