Convert CONFIG_NAND_ATMEL to Kconfig

This converts the following to Kconfig:
   CONFIG_NAND_ATMEL

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/sama5d3xek_nandflash_defconfig b/configs/sama5d3xek_nandflash_defconfig
index 972926d..b2e46e5 100644
--- a/configs/sama5d3xek_nandflash_defconfig
+++ b/configs/sama5d3xek_nandflash_defconfig
@@ -56,6 +56,8 @@
 CONFIG_DM_MMC=y
 CONFIG_GENERIC_ATMEL_MCI=y
 CONFIG_MTD_NOR_FLASH=y
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y