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/pm9263_defconfig b/configs/pm9263_defconfig
index 98b4385..8365638 100644
--- a/configs/pm9263_defconfig
+++ b/configs/pm9263_defconfig
@@ -35,6 +35,8 @@
 CONFIG_AT91_GPIO=y
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
+CONFIG_NAND=y
+CONFIG_NAND_ATMEL=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y