fdt: Correct cast for sandbox in fdtdec_setup_memory_size()

This gives a warning with some native compilers:

lib/fdtdec.c:1203:8: warning: format ‘%llx’ expects argument of type
   ‘long long unsigned int’, but argument 3 has type
   ‘long unsigned int’ [-Wformat=]

Fix it with a cast.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/fdtdec.c

@@ -1169,7 +1169,8 @@ int fdtdec_setup_memory_size(void)
 	}
 
 	gd->ram_size = (phys_size_t)(res.end - res.start + 1);
-	debug("%s: Initial DRAM size %llx\n", __func__, (u64)gd->ram_size);
+	debug("%s: Initial DRAM size %llx\n", __func__,
+	      (unsigned long long)gd->ram_size);
 
 	return 0;
 }