mtd: ensure UBI is compiled when using fastmap

UBI must be enabled when using fastmap, reflect this is defconfigs.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
diff --git a/configs/pxm2_defconfig b/configs/pxm2_defconfig
index 60907a1..817a492 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -70,6 +70,7 @@
 CONFIG_MTD_RAW_NAND=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
+CONFIG_MTD_UBI=y
 CONFIG_MTD_UBI_FASTMAP=y
 CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT=1
 CONFIG_MII=y