nios2: enable Driver Model

Enable the Driver Model config. The driver subsystems
are not enabled until we enable each of them.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Marek Vasut <marex@denx.de>
diff --git a/configs/nios2-generic_defconfig b/configs/nios2-generic_defconfig
index 09cc51e..2acf45d 100644
--- a/configs/nios2-generic_defconfig
+++ b/configs/nios2-generic_defconfig
@@ -1,4 +1,5 @@
 CONFIG_NIOS2=y
+# CONFIG_SYS_MALLOC_F is not set
 CONFIG_TARGET_NIOS2_GENERIC=y
 CONFIG_DEFAULT_DEVICE_TREE="3c120_devboard"
 CONFIG_HUSH_PARSER=y
@@ -13,3 +14,4 @@
 CONFIG_CMD_PING=y
 CONFIG_OF_CONTROL=y
 CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_DM=y