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/axm_defconfig b/configs/axm_defconfig
index 2e325d9..f1bdd0f 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -37,6 +37,8 @@
 CONFIG_OF_EMBED=y
 CONFIG_ENV_IS_IN_NAND=y
 # CONFIG_MMC is not set
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHYLIB=y