microblaze: Fix compilation failure because of missing libdts

Microblaze platform can use CONFIG_OF_EMBED option
but also it is necessary to support boards
which don't want to use this option.
U-Boot doesn't compile dts/libdts.o for #undef CONFIG_OF_EMBED
case that's why it should be guarded by ifdef.

Signed-off-by: Michal Simek <monstr@monstr.eu>