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>

arch/microblaze/cpu/u-boot.lds

@@ -45,7 +45,9 @@ SECTIONS
 	.data ALIGN(0x4):
 	{
 		__data_start = .;
+#ifdef CONFIG_OF_EMBED
 		dts/libdts.o (.data)
+#endif
 		*(.data)
 		__data_end = .;
 	}