configs: enable use of DT for all m68k boards

Enable DT usage for all m68k boards. To provide a
working single binary, the dts has been kept as embedded.

Signed-off-by: Angelo Dureghello <angelo@sysam.it>
diff --git a/configs/M54418TWR_serial_rmii_defconfig b/configs/M54418TWR_serial_rmii_defconfig
index a239f4a..7853992 100644
--- a/configs/M54418TWR_serial_rmii_defconfig
+++ b/configs/M54418TWR_serial_rmii_defconfig
@@ -1,6 +1,7 @@
 CONFIG_M68K=y
 CONFIG_SYS_TEXT_BASE=0x47E00000
 CONFIG_TARGET_M54418TWR=y
+CONFIG_DEFAULT_DEVICE_TREE="M54418TWR_serial_rmii"
 CONFIG_SYS_EXTRA_OPTIONS="CF_SBF,SYS_SERIAL_BOOT,SYS_INPUT_CLKSRC=50000000"
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="root=/dev/nfs rw nfsroot=192.168.1.1:/tftpboot/192.168.1.2  ip=192.168.1.2:192.168.1.1:192.168.1.1: 255.255.255.0::eth0:off:rw console=ttyS0,115200"