ARM: atmel: at91sam9n12ek: enable spl support

Enable SPL support for at91sam9n12ek boards, now it supports
boot up from NAND flash, serial flash.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
diff --git a/configs/at91sam9n12ek_nandflash_defconfig b/configs/at91sam9n12ek_nandflash_defconfig
index f908246..11b42d4 100644
--- a/configs/at91sam9n12ek_nandflash_defconfig
+++ b/configs/at91sam9n12ek_nandflash_defconfig
@@ -1,3 +1,4 @@
+CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9N12,SYS_USE_NANDFLASH"
 CONFIG_ARM=y
 CONFIG_ARCH_AT91=y