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/gurnard_defconfig b/configs/gurnard_defconfig
index 6c222d6..f00a06a 100644
--- a/configs/gurnard_defconfig
+++ b/configs/gurnard_defconfig
@@ -24,6 +24,8 @@
 CONFIG_CMD_FAT=y
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_IS_IN_NAND=y
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_PHYLIB=y
 CONFIG_TIMER=y
 CONFIG_ATMEL_PIT_TIMER=y