microblaze: Dont enable UBI support by default

This patch removes UBI support from defconfig and it can
be enabled from menuconfig as per need.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>