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/at91sam9g10ek_dataflash_cs0_defconfig b/configs/at91sam9g10ek_dataflash_cs0_defconfig
index 0caede5..632031a 100644
--- a/configs/at91sam9g10ek_dataflash_cs0_defconfig
+++ b/configs/at91sam9g10ek_dataflash_cs0_defconfig
@@ -35,6 +35,8 @@
 CONFIG_DM_GPIO=y
 CONFIG_AT91_GPIO=y
 # CONFIG_MMC is not set
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y