ARM: OMAP: omap2plus_defconfig: Enable EXT4 support

On OMAP boards that includes an SD card slot, an EXT4 partition could
be used to store the root file system. So, the kernel should have
built-in support for EXT4 to be able to mount the VFS root on boot.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>