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/at91sam9x5ek_mmc_defconfig b/configs/at91sam9x5ek_mmc_defconfig
index 01c5818..3a2d6f4 100644
--- a/configs/at91sam9x5ek_mmc_defconfig
+++ b/configs/at91sam9x5ek_mmc_defconfig
@@ -39,6 +39,8 @@
 CONFIG_AT91_GPIO=y
 CONFIG_DM_MMC=y
 CONFIG_GENERIC_ATMEL_MCI=y
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y