microblaze: defconfig: Disable EXT2 driver and Enable EXT3 & EXT4 drivers

As EXT4 filesystem driver is used for handling EXT2 file systems as
well. There is no need to enable EXT2 driver. This patch disables EXT2
and enables EXT3/EXT4 drivers.

Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>