nios2: use cfi flash driver model

Use cfi flash driver model.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
diff --git a/configs/nios2-generic_defconfig b/configs/nios2-generic_defconfig
index e42a15e..0a6de6f 100644
--- a/configs/nios2-generic_defconfig
+++ b/configs/nios2-generic_defconfig
@@ -17,6 +17,8 @@
 CONFIG_ALTERA_PIO=y
 CONFIG_MISC=y
 CONFIG_ALTERA_SYSID=y
+CONFIG_MTD=y
+CONFIG_CFI_FLASH=y
 CONFIG_DM_ETH=y
 CONFIG_ALTERA_TSE=y
 CONFIG_ALTERA_JTAG_UART=y