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/sama5d3_xplained_nandflash_defconfig b/configs/sama5d3_xplained_nandflash_defconfig
index b72462d..4efcba6 100644
--- a/configs/sama5d3_xplained_nandflash_defconfig
+++ b/configs/sama5d3_xplained_nandflash_defconfig
@@ -54,6 +54,8 @@
 CONFIG_AT91_GPIO=y
 CONFIG_DM_MMC=y
 CONFIG_GENERIC_ATMEL_MCI=y
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_DM_ETH=y
 CONFIG_MACB=y
 CONFIG_PINCTRL=y