mtd: atmel_nand: add missing include

using this driver in SPL code with CONFIG_SPL_NAND_ECC
configured leads in an compileerror. Fix this.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Reviewed-by: Bo Shen <voice.shen@atmel.com>
[fix subject]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>

drivers/mtd/nand/atmel_nand.c

@@ -18,6 +18,7 @@
 #include <malloc.h>
 #include <nand.h>
 #include <watchdog.h>
+#include <linux/mtd/nand_ecc.h>
 
 #ifdef CONFIG_ATMEL_NAND_HWECC