configs/qemu_or1k_defconfig: enable ethernet

The issue seems to be fixed. I checked qemu and linux git repositories
but can not find any information when this is starting to work.

System gets successfully an IP, no further testing was done.
Tested with Qemu 8.0.3 and Linux 6.1.x.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/qemu/or1k/readme.txt

@@ -3,5 +3,3 @@ Run the emulation with:
  qemu-system-or1k -kernel output/images/vmlinux -nographic # qemu_or1k_defconfig
 
 The login prompt will appear in the terminal that started Qemu.
-
-Ethernet support is not working, yet.

configs/qemu_or1k_defconfig

@@ -1,6 +1,9 @@
 # Architecture
 BR2_or1k=y
 
+# System
+BR2_SYSTEM_DHCP="eth0"
+
 # Filesystem
 # BR2_TARGET_ROOTFS_TAR is not set
 BR2_TARGET_ROOTFS_INITRAMFS=y