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_mmc_defconfig b/configs/sama5d3_xplained_mmc_defconfig
index 861a851..3e8cf67 100644
--- a/configs/sama5d3_xplained_mmc_defconfig
+++ b/configs/sama5d3_xplained_mmc_defconfig
@@ -57,6 +57,8 @@
 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