microblaze: Build only DTBs for selected target

Adding more targets to repository requires some additional
changes not simply just adding config file, defconfig and dts.
This patch makes this process easier by building only
particular DTB which is selected via defconfig
that Makefile doesn't need to contain all dts files in the repository.

Reported-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>