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/at91sam9g20ek_nandflash_defconfig b/configs/at91sam9g20ek_nandflash_defconfig
index b6affb8..d6bcf69 100644
--- a/configs/at91sam9g20ek_nandflash_defconfig
+++ b/configs/at91sam9g20ek_nandflash_defconfig
@@ -34,6 +34,8 @@
 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