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/wb50n_defconfig b/configs/wb50n_defconfig
index e6b6baa..ea27852 100644
--- a/configs/wb50n_defconfig
+++ b/configs/wb50n_defconfig
@@ -24,6 +24,8 @@
 CONFIG_CMD_PING=y
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y