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/M5253DEMO_defconfig b/configs/M5253DEMO_defconfig
index 4685c4e..2dbbd22 100644
--- a/configs/M5253DEMO_defconfig
+++ b/configs/M5253DEMO_defconfig
@@ -1,6 +1,7 @@
 CONFIG_M68K=y
 CONFIG_SYS_TEXT_BASE=0xFF800000
 CONFIG_TARGET_M5253DEMO=y
+CONFIG_DEFAULT_DEVICE_TREE="M5253DEMO"
 CONFIG_BOOTDELAY=5
 # CONFIG_DISPLAY_BOARDINFO is not set
 # CONFIG_CMDLINE_EDITING is not set