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>