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>

configs/nios2-generic_defconfig

 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_DHCP=y
 CONFIG_CMD_PING=y
 CONFIG_OF_CONTROL=y
 CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_DM=y