ARM: vexpress_*_defconfig: Enable CMD_UBI support

This allow for convenient use of QEMU machine to test loading of UBI
filesystem. There are a couple of changes made together of this which
are required:

 1) The malloc must be at least 512 KiB to allow the use of UBI
    filesystem. We are going to enable it in a next patch.

 2) MTD_DEVICE must be enabled, otherwise we get missing symbols
    during the build

Following configs were change:

 - vexpress_aemv8a_dram_defconfig
 - vexpress_aemv8a_juno_defconfig
 - vexpress_aemv8a_semi_defconfig
 - vexpress_ca15_tc2_defconfig
 - vexpress_ca5x2_defconfig
 - vexpress_ca9x4_defconfig

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
diff --git a/configs/vexpress_ca15_tc2_defconfig b/configs/vexpress_ca15_tc2_defconfig
index f4d555b..cabc0c4 100644
--- a/configs/vexpress_ca15_tc2_defconfig
+++ b/configs/vexpress_ca15_tc2_defconfig
@@ -18,8 +18,10 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NFS is not set
 # CONFIG_CMD_MISC is not set
+CONFIG_CMD_UBI=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_MTD_NOR_FLASH=y
+CONFIG_MTD_DEVICE=y
 CONFIG_FLASH_CFI_DRIVER=y
 CONFIG_SYS_FLASH_USE_BUFFER_WRITE=y
 CONFIG_SYS_FLASH_PROTECTION=y