Convert CONFIG_MTD_PARTITIONS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_MTD_PARTITIONS
   CONFIG_MTD_DEVICE

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/at91sam9n12ek_nandflash_defconfig b/configs/at91sam9n12ek_nandflash_defconfig
index 1627206..ac07bcb 100644
--- a/configs/at91sam9n12ek_nandflash_defconfig
+++ b/configs/at91sam9n12ek_nandflash_defconfig
@@ -39,6 +39,7 @@
 CONFIG_DM_MMC=y
 CONFIG_GENERIC_ATMEL_MCI=y
 CONFIG_NAND_ATMEL=y
+CONFIG_MTD_PARTITIONS=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y