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/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index 9954be5..073e855 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -26,5 +26,7 @@
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_CMD_PCA953X=y
 # CONFIG_MMC is not set
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y