microblaze: Fix FDT message in boot_prep_linux()

There is no need to show FDT message in regular flow that's why switch it
to debug level.

Fixes: 09050460 ("microblaze: Switch to generic bootm implementation")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/lib/bootm.c

@@ -92,7 +92,7 @@ static void boot_jump_linux(bootm_headers_t *images, int flag)
 static void boot_prep_linux(bootm_headers_t *images)
 {
 	if (IMAGE_ENABLE_OF_LIBFDT && images->ft_len) {
-		printf("using: FDT\n");
+		debug("using: FDT\n");
 		if (image_setup_linux(images)) {
 			printf("FDT creation failed! hanging...");
 			hang();