microblaze: fix race condition in building boot images

I fixed a race condition in the parallel building of ARM in commit
3939f334 ("ARM: 8418/1: add boot image dependencies to not
generate invalid images").

I see the same problem for MicroBlaze too.

"make -j<N> ARCH=microblaze all linux.bin.ub" results in a broken build
because two threads descend into arch/microblaze/boot simultaneously.

Add proper dependencies to avoid it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>