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_mmc_defconfig b/configs/sama5d3xek_mmc_defconfig
index 37f0eb9..dcc7ed8 100644
--- a/configs/sama5d3xek_mmc_defconfig
+++ b/configs/sama5d3xek_mmc_defconfig
@@ -61,6 +61,8 @@
 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